- Under constant strain, there is a decreased stress with time in viscoelastic materials, which is called stress relaxation. Textiles experience various long lasting deformations during manufacture and application. Consequently, stress relaxation occurs in these materials. This phenomenon can cause disturbances in textile performance in technical applications such as surgical mesh, pressure garments, varicose stockings, pressure bandages, etc. Thus, by considering the factors affecting stress relaxation of the fabric, the ability to design and produce appropriate products increases. In the present study, net warp knitted fabrics with five different structures including Tricot, Pin hole-net, Sandfly, quasi-Sandfly, and quasi-Marqussite have been produced and the effect of fabric structure on the stress relaxation of the fabrics in the course and wale directions have been investigated. To investigate the stress relaxation of the fabrics, a new index, named stress relaxation index was introduced. This index is obtained by multiplication of initial stress by the porosity of the fabric divided to the mass per unit area of the fabric. The results demonstrated that fabric structure has remarkable effect on the stress relaxation of the fabrics, and by increasing the stress relaxation index, stress relaxation of the fabric in both directions increases. Fabrics with Pin hole-net and quasiMarqussite structures showed the highest and lowest stress relaxation in the course direction, respectively. Meanwhile, fabrics with Tricot and Sandfly structures exhibited the highest and lowest stress relaxation in the wale direction, respectively.