Behavior of spherical metal particle was observed with high speed camera at the instance of partial discharge inception in oil immersed insulating system. The polarity of metal particle's charge was evaluated by the behavior of the particle and the direction of electric field. The positively charged particle moves to the press-board on negative electrode to initiate partial discharge. The charge quantity of the particle can be calculated by the behavior of particles and electric fields. The movement of particle is so notable on AC peak voltage that partial discharge initiation can be observed on the peak voltage. The mechanisms of partial discharge is proposed.