As the prime after-treatment device for diesel particulate matter (PM) emission control, Diesel Particulate Filter (DPF) has been widely used for its high particle capture efficiency. In order to study the particle motion and deposition distributions in the DPF inlet channel, a 2-D wall flow DPF microcosmic channel model is built in this paper. The motion trajectories of particles with different sizes are investigated considering the drag force, Brownian motion, gravity and Saffman lift. The effects of the space velocity on particle motion trajectories and deposition distributions inside the inlet channel are evaluated. These results demonstrate that the particle motion trajectories are highly dependent on particle sizes and influenced by the space velocity. The effect of the Brownian motion is obvious for fine particles and suppressed when the space velocity is raised. Moreover, various patterns of particle deposition distributions at the wall surface along the inlet channel are obtained under different particle sizes. The uniformity of particle deposition distributions is closely related to the particle size. With the increase of the particle size, more particles deposit at the rear-part of the inlet channel. Finally, in order to estimate the uniformity magnitude of particle deposition distributions, a uniformity index is proposed and discussed in this paper.