Abstract: The transmission performance of a harmonic gear lubricated with 601EF grease was investigated under the vacuum thermal cycling environment. The results show that the lubricating properties of the harmonic gear are directly influenced by the temperature of the grease, the load and the rotary speed of the harmonic gear. Under the low temperature condition, the lubricating properties of the harmonic gear deteriorate significantly due to the viscosity rise of 601EF grease.