Effect of obstructions on the particle collection efficiency in the collecting stage of a two-stage electrostatic precipitator was studied. The electric field was obtained by solving the Laplace equation and the turbulent gas velocity profile was given from the standard k-epsilon turbulence model. The trajectory model was adopted to obtain the information on the particle phase trajectories and number concentrations. The equation of particle motion included inertial, viscous drag and electrostatic force. The Stokes number (the ratio of inertia to viscous drag force) and electrostatic number (the ratio of electrostatic force to viscous drag force) were introduced for interpreting the particle transport in an obstructed electrostatic precipitator. It was observed that larger particles were not entrained into the recirculation zones. Especially, the obstructions increased the collection efficiency for the smaller particles. As the Stokes number increased, the total efficiency did not increase monotonically and had a minimum efficiency point. Consequently, the presence of the obstructions does not always deteriorate the particle collection rate in a two-stage electrostatic precipitator.