The Gaussian doping is used to optimize the performance of InP/InGaAs uni-traveling-carrier photodiode (UTC-PD) in this letter. The UTC-PD structure is modeled with drift-diffusion approach and the comparisons of the characteristics for four UTC-PDs with different doping schemes in absorption layer are made. According to the comparison, one optimized UTC-PD where the InP collection layer is partly replaced by a depleted InGaAs using Gaussian doping on top of lightly constant background doping in the absorption layer is presented, with f3dB of 79 GHz, which is more than 1.9 times higher than that with the constant doping in the absorption layer.