In order to provide the optimal design parameters, we have investigated the characteristics of a combination of metal plate and phosphor screen, a typical X-ray sensor in electronic portal imaging system, by using Monte Carlo simulation. Detection efficiency and spatial resolution of the X-ray sensor were estimated based on the bremsstrahlung spectrum of the incident X-ray and for various thicknesses of metal plate and phosphor screen. The detection efficiency was calculated from the total absorbed energy and the spatial resolution was defined from the spatial distribution of the absorbed energy. Simulation results showed a trade-off relationship between the detection efficiency and the spatial resolution. It was also found that the detection efficiency and the spatial resolution were mainly determined by thickness of metal plate and phosphor screen, respectively. The simulation will be useful when determining the optimal thickness of phosphor screen as well as of metal plate for the sensor design of therapeutic X-Pay imaging. (C) 1999 Elsevier Science B.V. All rights reserved.