In this research work, behavior of flexural stiffness of core spun cotton spandex single jersey, $1{ imes}1$ rib and interlock fabrics was studied under relaxation and machine washing treatments. Results are compared with similar fabrics made from 100 % cotton. Fabric weight density increased with the progression of treatments and it is proportionate to the fabric tightness factor (stitch $length^{-1}$). Even though both types of fabrics had same machine set stitch lengths, cotton/spandex fabrics have shown the higher fabric weight densities than that of 100 % cotton fabrics. Although $1{ imes}1$ rib and single jersey fabrics knitted with the same machine set stitch lengths, rib fabrics have given higher fabric weight densities than single jersey fabrics. Among the three knitted structures, interlock fabrics with higher machine set stitch lengths gave the higher fabric weights. Fabric stiffness and flexural rigidity have given higher values under the progression of treatments and it was found that higher values of stiffness have given by cotton/spandex knitted fabrics compared to their cotton fabrics. Fabric stiffness and flexural rigidity in wale direction were higher than that in course direction, but it is only observed in single jersey fabrics. However, $1{ imes}1$ rib and interlock fabrics have shown an opposite behavior. It was also observed a positive correlation between TF (i.e.: stitch $length^{-1}$) and bending length/flexural rigidity in both fabric types. Lower flexural rigidities reported with single jersey structures and highest values gave with interlock structures of cotton/spandex and cotton fabrics.