Abstract: It was observed that the polyphase liposomes(PL), as drug carriers, were able to stablize MTX chemically. The shelf lives of MTX in PL and in phosphate buffer solution were 4.2 and 3.5 years, respectively.A new route of MTX degradation was found when-MTX was kept in phosphate buffer solution at 25℃ for 3 months in the absence of light. The reaction was catalyzed by phosphate, but inhibited by PL and high temperature (60℃).