The optimum thickness of metallic thin film utilized to support the surface plasmon resonance (SPR) has been investigated, based on the characteristics of electromagnetic field energy distributing in the film and its compound permittivity. It was shown that the optimum film thickness is related to the wavelength of exciting light and the refractive index (RI) of the metallic thin film. In this paper, a mathematical expression has been established to describe their relationship, and verified in the experiment. Our theoretical analysis is also consistent with previous experimental results. When the SPR sensor is employed in angular interrogation, in order to obtain highest sensitivity, the optimum thickness of metallic thin film can be deduced from the wavelength of the exciting light and the imaginary part of the film RI, or be obtained with the center wavelength of the exciting light and imaginary part of RI when the sensor is utilized in wavelength interrogation.