This paper reports the trajectories of contaminant particles in the charge-injection type of electrostatic oil filter, and the effects of the supply flow velocity, the oil viscosity, and the filter element shape on the particle collection efficiency are investigated. The flow and electric fields in the filter are numerically simulated based on an assumption about the charge injected by the emitter electrode. Then, the particles' trajectories are calculated by numerically solving their equations of motion with different initial positions at the inlet of the filter. The simulated particle collection efficiency is compared with its measured value. The numerical results agree qualitatively with the experimental results. This particle trajectory analysis will be useful for improving the structure of the charge injection type of electrostatic oil filter.