A new optimized structure of an UTC (uni-traveling-carrier) photodiode is developed and epitaxied by metal-organic chemical vapor deposition. We fabricated a UTC photodiode of 30 μm in diameter. Theoretical simulation based on drift-diffusion model was used to analyze the space-charge-screening effect in UTC photodiode primarily in two aspects: the carrier concentrations and the space electric field. The simulation results were generally in agreement with the experimental data.