Abstract: Impacts of fatty acyl cystine on the biodegradability and tribological performances of mineral lubrication oil were investigated based on the biodegradation and tribological experiments; the mechanism for additive to promote the biodegradability of lubricant was also discussed by the observation on morphous change of bacteria in the process of paraffin oil degradation. The results showed that fatty acyl cystines significantly improved the biodegradability of paraffinic base oil, and exhibited good load-carrying capacity, friction reduction and antiwear properties. The B1502 bacteria started to form spores after 12 h, and all the bacteria transformed into spores after 48 h while growing in the medium with paraffin oil as unique carbon source. However, the bacteria might still maintain the morphous of trophozoite and few spores was observed in the test period of 168 h while fatty acyl amino acid was added into the same medium. The mechanism for additive promoting the biodegradability of lubricant was that fatty acyl amino acid could furnish enough nutrient content for degradation bacteria in the initial stage, which might keep the bacteria growing vigorously, and improve the expression level or activity of enzymes related to degradation of paraffin oil, which resulted in the enhancement of biodegradability of paraffin lubricating oil.