The third-harmonic voltages at the generator neutral and the terminals， and the phase angle relationship between fundamental frequency zero-sequence voltage and zero-sequence current under different single-phase grounding faults are analyzed. According to that， a phase-angle difference direction element for generator stator grounding fault protection is presented. It consists of two parts: for fault location less than 50% of stator winding from neutral point， the direction function is based on the phase comparison of the third-harmonic voltages at the generator neutral with that at the terminals. But it cannot detect the ground fault direction near the generator terminal（for fault location larger than 50% of stator winding from neutral point）， which can be determined by the phase comparison of the fundamental frequency zero-sequence voltage with fundamental frequency zero-sequence current. Research results show the direction element can detect stator grounding faults with high sensitivity and accuracy.