Abstract: In this paper,a new test method for anti-fogging performance of greenhouse film of polyethy lene basedon the determination of surface tension was proposed. The relationship between the surface tension andtime of invalidation was formulated. A critical surface tension(34x10^-3N/m) was set, below whichthe film was considered as invalidating During the test, the film was immersed in hot wa- terat 50℃ and the time lasted for the surface tension of the film decreased below the critical value was measured,which was considered measure of the time of invalidation.