Abstract: Diffraction efficiency investigation of zone plate with stepped phase for terahertz frequencies was presented in this paper. According to Rayleigh-Sommerfeld diffraction theory, the zone plate with phase stepped changing can focus Gaussian beam with high diffraction efficiency. Diffraction law of the phase-stepped-changing zone plate was similar to thin lens. Simulations of intensity distribution and diffraction efficiency in phase-stepped-changing, phase-continuous-changing and phase-correcting Fresnel zone plate were conducted and compared. Results show that the focusing performance and the diffraction efficiency of phase-stepped-changing zone plate is better. A phase-stepped-changing zone plate is calculated. It can be produced by micro-mechanical method.