One of the most important mechanical properties of a fabric is tensile strength, which means that it has the resistance capacity against the external forces without tearing. Tensile strength of fabrics is evaluated as uniaxial, biaxial and multiaxial. It is so important to investigate the tensile strength of fabrics that are applied in two perpendicular directions simultaneously, as fabrics are used as garments or reinforcer in the composite industry, are exposed to such forces when consumed. This research focuses on the biaxial tensile behavior of weft- knitted fabrics which are widely used in the field of garments. For this purpose, weft- knitted fabrics with simple rib structure, three simple interlock structures and Half-millano interlock and Half-cardigan interlock were produced to evaluate uniaxial and biaxial tensile behavior. Then a biaxial tensile test machine was designed. This device operates with one load-cell and can be connected to a uniaxial tension machine INSTRON. Finally, the effect of fabric structure was investigated on uniaxial and biaxial tenile behaviors. The strain variations in two wale and course directions per unit time were used as two indices to evaluate the biaxial tensile properties. Also, Poisson coefficient changes of the fabrics were measured and compared for each of these structures relative to time. Also, The strain ratio of the wals to the course were calculated for each of these structures under a biaxis tensile test.