The investigations on the photothermal properties of nanoparticles present the following two problems. Firstly, approximate models and theories are used to calculate the optical properties of nanoparticles. Secondly, the optimal size parameters of the nanoparticles that correspond to the best photothermal properties are not given. To solve the above-mentioned problems, the effects of the light wavelength and the particle size on the photothermal properties of Au nanoshells and nanospheroids were analyzed by using the time-dependent heat transfer equation and the accurate light scattering theory (Mie theory and T-matrix). The maximum steady-state temperature changes at four different typical wavelengths (800 nm, 808 nm, 820 nm, and 1064 nm) in the near-infrared region and the corresponding optimal particle sizes were obtained. The optimized Au nanoparticles can be used as an ideal heat source in photothermal therapy.