The drawbacks of present stator ground fault protection principle for the small and medium sized generators are pointed out. Then the characteristics of direction relation between zero sequence current and zero sequence voltage are discussed when system single-phase ground fault or stator ground fault occurs under the circumstances of neutral ground via Petersen coil or neutral non-ground. A new stator ground protection principle is proposed based on the direction relation between zero sequence current and zero sequence voltage as well as the direction relation between zero-sequence differential current and zero sequence voltage. Tests and field operation demonstrate the feasibility of the principle proposed.