Since a large demand has been placed on building material industry especially in the last decade, the civil structures may be established on weak subgrades. Therefore, soil reinforcement is important as a technique to improve the engineering characteristics of soil. In this way, using natural fibers to reinforce soil is an old and ancient idea. Consequently, randomly distributed fiber-reinforced soils have recently attracted increasing attention in geotechnical engineering for the second time. However, balling, clumping and folding are the three major problems involved with the short fiber composite soil production. The main aim of this research was using loop-formed fibers made from conventional geogrids to reinforce soil. The soil used in this study was a problimatic silty sand type prepared from Segzi desert. In the experimental section, a set of laboratory direct shear tests and compaction tests were performed on different samples including both neat soil, loop-formed fiber and fiber reinforced treatments. As well, a novel apparatus based on fiber pull-out test was designed to determine the interfacial shear stress between fiber and soil by modifying the Instron Tensile Tester. The theoretical section which contains two major parts includes micro and macro mechanical analyses. In both methods shear behavior of fiber and loop-formed fiber reinforced soil composite was modeled for all treatments. Consequently, the reinforced soil was considered as a soft-matrix composite. The proposed macro-mechanical analysis was based on "force-equilibrium method". Thus, the presented model demonstrated that shear resistance of loop-formed fibers has two major components including "fiber effect" and "loop effect". The macro-mechanical modeling was performed on both cases of "pure slippage" and "relative slippage". In the later method, a semi-empirical method was used to predict the shear strength of soil composites. Therefore, an artificial neural network (ANN) technique was established on the experimental data obtained from direct shear tests. In addition, least square error (LSE) method was generalized to solve two-variable equations. In micro-mechanical method, shear strength of fiber reinforced soil composite was investigated by using finite element method (FEM). Direct shear test was simulated through Abaqus 6.9 software. The presented model demonstrated that in the case of fiber reinforced soil composite, the reinforcing phenomenon is related to the performance of vertical fibers; meanwhile both vertical and horizontal fibers forming loop elements and/or discreet grids are involved in shear resistance of loop-formed fiber reinforced soil composite. This concept has been explained by the “loop effect”. Comparison of the theoretical and experimental results revealed that the macro-mechanical analysis based on relative slippage method and the micro-mechanical analysis are able to predict the shear strength of the soil composite. The model error of the macro-mechanical analysis using pure slippage is relatively high, but the trend of the results is well rellated with the experimental outputs. On the whole, at the same orientation, fiber content, fiber finesse and fiber length, shear strength of composite soil in the case of looped fiber reinforced samples is higher than that of fiber reinforced treatments which could be due to “loop effect”.