Shear modulus of polyacrylamide (PAAM) gels has been measured with the network concentration ranging from 0.02 to 1.0 g/cm(3). In the low concentration region, the shear modulus is a scaling function of network concentration with the exponent equal to 1/3, which is expected by classical rubber elasticity theory. As the concentration increases, the behaviour of shear modulus deviates the scaling law significantly. This deviation has been observed from all six series of gels with cross-linking concentrations varying from 0.61 to 4.91 mol%. The modified Mooney theory which incorporated a second higher order term is used to explain the results. This second term is believed to be a result of gel inhomogeneity. The measured shear modulus data can be fitted well with the new theory, with two exponents equal to 0.33 and around 2.0-2.5, respectively. The crossover concentration from the classical behaviour to the non-classical behaviour was found to be cross-linking concentration dependent.