From the stress-strain curves of geomembranes, initial modulus, yield point, tensile strength and breaking elongation were obtained. It was derived from a relationship between the initial modulus and yield point by using the mathematical model. The order (n) of parabolic equation, $sigma$=$sigma$${gamma}$(1-(($varepsilon$${gamma}$-$varepsilon$)/$varepsilon$${gamma}$)"), is established to represent theoretically the uniaxial tensile behavior of geomembranes. The mechanical properties of HDPE, ECB and EVA geomembranes were examined according to ASTM D638 and substituted the experimental data for theoretical parameters. After determining (n), 2% secant modulus as an elastic modulus is considered to obtain the index value. It is proved that the initial modulus is equal to n times of secant modulus at yield point. When n is 4 (HDPE) and 7 (ECB, EVA), respectively, the theoretical values of mathematical model are located within the deflection of expermental data. And it is thought that 2% secant modulus, as an index modulus, is more appropriate than the initial modulus(tangent modulus at origin), because the 2% secant modulus has narrower error range than the initial modulus.ulus.