Abstract: With the aid of a semi-analytic treatment, the performance of rod-pinch diode with or without ions was characterized via particle-in-cell simulations. With the increase of diode current, the sequential transitions of the space-charge-limited flow, weakly pinched flow and magnetically insulated flow were clearly shown from electrons position and space current flow patterns. It is concluded that critical magnetically insulated current can be reached at lower voltage when ion emission is allowed, and only with ions, electrons can propagate along the anode surface and pinch can form at the tip of the rod; ions are necessary for rod pinch diode which can provide a small-area, high-yield X-ray source for pulsed radiography.