RF performances of double-drift InP-based Impatt diode with and without optical injection at Terahertz (THz) frequencies have been studied by using computer simulation experiment developed by the authors. A double iterative computer simulation method based on modified drift-diffusion model has been used to study the performance of the diode at THz regime. The breakdown voltage and dc-to-RF conversion efficiency for InP DDR IMPATT at 0.3 THz is higher than 0.5 THz frequency. The study indicates that the InP IMPATT diode is capable of generating CW RF power of 41.7 mW and 27.0 mW at 0.3 THz and 0.5 THz frequencies respectively. In this model, the ionization coefficients of both electrons and holes are fitted with the experimental data. Due to the parasitic resistance, the value of negative conductance and RF power of the unilluminated diode is reduced by 7.5%. The current multiplication factor for both electrons and holes are changed to study the leakage current in controlling the performance of the device. The design results and proposed experimental methodologies are described in this paper will be helpful to realize InP IMPATT as optically controlled THz switch. © 2016 American Scientific Publishers. All rights reserved.