Particle deposition in duct air flow over a forward- or backward-facing step was investigated by Reynolds stress model with velocity fluctuation correction combined with discrete particle model. The mean air velocities in duct with a backward-facing step and particle deposition velocities for smooth duct both agreed well with related literature results. The deposition velocities of the duct with a forward-facing step are significantly higher than those of smooth duct, especially for small particles (d(p) < 10 mu m). Particle deposition velocities in the duct with a backward-facing step are higher for small particles (d(p) < 10 mu m) but lower for large particles (d(p) > 10 mu m), compared with smooth duct case. Air velocity and turbulent kinetic energy fields in the duct with a forward- or backward-facing step are quite different from smooth duct case, which modify the particle deposition characteristics significantly. Furthermore, a particle deposition efficiency ratio was defined and analyzed by considering flow drag of the duct. The results showed that the peak deposition efficiency ratio of the duct with a forward-facing step can reach 60 for particles of 1 The duct with a forward-facing step can enhance deposition of small particles more significantly (d(p) < 10 mu m), compared with the duct with a backward-facing step.