Tensile fabric structures which are also known as tensile membrane structures are a part of the big category of tensile structures due to their specific tensile behavior under forces. Moreover they also belong to the category of spatial structures because of possessing a three-dimensional geometric shape and additionally their low weight makes them a part of lightweight structures. The use of tensile fabric structures in designing stadiums, shopping centers, parking lots, etc. provides flexibility in the matter of choosing the structural form to a greater extent. All over the world, the most astonishing and complex designs are becoming possible due to the lightness and flexibility of the consumed materials. One important issue in the process of designing these structures is the close relation between the form and the structural behavior which indeed requires the close cooperation of Structural engineers and Architects. Therefore one of the most critical steps in the process of designing the tensile fabric structures is to find a form which simultaneously meets both structural engineers’ and architects’ expectations. Because of the limited research papers in Persian literature in the area of the design of tensile fabric structures, which these limited number of publications mostly have focused on the architectural knowledge of tensile fabric structures, this thesis aims to provide an underlying study as a groundwork for future studies in preliminary form design, analysis and construction of these spatial structures. Therefore besides finding a preliminary form for these structures, which the focus of this thesis is mostly on, the other steps in designing of these structures alongside with a summary of the history of the studies (followed by a brief explanation) is also included in this thesis. Furthermore with the examination of the existing methods for preliminary form design and considering different concerns and obstacles which architects would face in achieving a form for these spatial structures, some of these concerns were addressed with a structural approach. In order to resolve these concerns, a computer program has been developed in which it doesn’t require a strong knowledge of structural engineering that makes it suitable for those architects who do not poses a comprehensive structural engineering background which provides them with a means of obtaining the ideal plausible form. It has been intended to make the computations as less time and money costly as possible. As in an architect’s point of view, one of the most important factors in choosing the form is the way in which the structural form relates to the surrounding environment, the program’s output is provided in an accepted format for common designing software which enables the architect to analyze the acquired structural form in close comparison with three-dimensional architectural elements. Another obstacle for architects is their lack of knowledge regarding the existing methods for achieving different forms, so factors that affect the preliminary form such as internal forces magnitude, boundary conditions and methods of defining the elements have been investigated. Changing any of these factors will create new forms. The results will enable the architect to get familiarized with the changes that these factors produce and to use them as a means of achieving the different forms. In order to achieve a stable structural form, the force density method has been applied throughout the studies. In this method computations are done with the use of bar elements and by satisfying the stability equations a set of stable forms under pre-stressed forces will be acquired according to the boundary conditions. Furthermore by the use of repetitive methods such as Multi-step force-density method with stress adjustment (MFDS) and Multi-step force density method with force adjustment (MFDF), a form has been attained in which all the elements have the same internal forces. This is done to prevent the formation of wrinkles on the structure’s surface and to minimize the area/amount of consumed fabric materials. Eventually, the NURBS (Non-Uniform Non Rational B-Spline) have been implemented in generating the program’s output in order to reduce the computational time and to provide a more appropriate means of implementing the attained geometric form in common design software. Keywords : Tensile fabric structures, Preliminary form finding, force density method, multi-step force density methods, NURBS(Non-Uniform Non Rational B-Spline.