In this paper, lead magnesium niobate–lead titanate (PMN–PT) piezoelectric single crystal and its 1–3 composite counterpart were characterized and analyzed under different stable temperatures using both a Simulated Annealing (SA) optimization algorithm and the commercial software PRAP (Piezoelectric Resonance Analysis Program). Electrical impedance resonance characteristics of the two material samples over the range 25–125 °C were measured. The correlation between experimental data and numerical fits derived from both SA and PRAP is considered. Calculation of the determination coefficient (Symbol) between numerically fitted and measured results is above 95% for both methods. Furthermore, variations in the number of data values used for the fit introduced no more than 3.1% uncertainty on the calculated material parameters. It is found that the complex material parameters of PMN–PT composite are more dependent on temperature than the single crystal. The phase transition of the PMN–PT, which is close to 90 °C, has an effect on the high temperature material characteristics of both piezoelectric materials. These calculated complex material parameters can be used for the design of ultrasonic transducers for elevated temperature applications.