The highly nonlinear extension curves of a PTFE-coated plain-weave glass fiber fabric were measured under three cases of biaxial stress ratio condition. A 4-step-linear approximation which uses least square method was applied on the extension curves. Then the elastic parameters were determined for each of the approximated 4-step-lines, and the 4-step-lines tracked on the measured curves satisfactorily. Further, using the elastic parameters, the deflection of a rectangular flat specimen (40×30 cm) subjected to pressure was analyzed.