The multivariable power dependence of polymer properties on molecular characteristics (Dobkowski, 1981) has been applied to molecular weight dependence of tensile strength, and the known equation of Flory (1945) has been extended taking polydispersity of polymers into account. Constant parameters of the relevant regression equations have been calculated using experimental data on tensile strength and molecular weight averages M(n) and M(w) of polystyrene (PS) and polycarbonate (PC). Then, the critical molecular weight for entanglements M(c) has been obtained from the following relationship: A = K sigma(infinity)M(c), where A and sigma(infinity) are parameters of the extended Flory equation for the tensile strength, and the constant K = 2 is assumed for linear polymers. It has been found that M(c) of injection and compression moulded PS is equal to 34000 and 37350 g/mole, respectively, while M(c) of injection moulded PC equals to 5000 g/mole. The values of M(c) calculated From the polymer tensile strength are consistent with published data obtained by other methods and with the computer modeling calculations. Branched polymers have only qualitatively been discussed. Dimensionless equations have been proposed for tensile strength characteristics for polymer materials. The described procedure can be suggested as applicable to various polymers for the determination of their M(c) values. However, more experimental data on another polymer materials will be necessary to support hitherto obtained results.