Abstract: To study the failure mechanism of harmonic drivers under the space environment, and provide a theoretical foundation for its reliability life testing, a 5 000 h vacuum life test of harmonic drivers was conducted. The results show that the flexspline-circular spline interface with DLC film/grease(Braycote601)composite lubricated operates well, and flexspline-wave generator bearing interface appeared serious wearing. Then numerical analysis of mixed lubrication was conducted to study the failure mechanism. The results showed that at 3～150 r/min range of speed, this contact area was always in mixed lubrication state, and which caused wear. The load proportion that asperity contact undertook was decreased with increasing of revolution, and was increased with increasing of temperature and load. At low speed , the temperature and load had great effect on asperity contact, and it was getting weaken with the increasing of speed.