A series of polyurethane elastomers with different hard-segment contents of 24% (M24), 34% (M34), 50% (M50), and 100% (M100) have been investigated through thermogravimetric analysis (TGA) measurement and mathematical simulation. The fourth-order Runge-Kutta method was used to solve the differential equations, and the solutions were obtained by running a written computer program. The activation energy and the frequency factor of polyurethane decomposition can be obtained by only one nonisothermal measurement of TGA. The same values of the activation energy and the frequency factor were obtained using the different constant heating rates under air for the elastomer with a hard-segment content of 34%. The activation energy decreases as the hard-segment content of polyurethanes increases. The polyurethane materials can be used at room temperature for several millennia without failure through lifetime calculation. Comparison of the values of t(1/2), the heat stability of polyurethanes decreases in the following order : M24 > M34 > M50 > M100; this phenomenon can be explained due to the heat instability of the urethane group. The highest concentration of the urethane group in the M100 structure results in the worst heat stability.