Abstract: Based upon a view that mass loss just is a diffusion releasingprocess,considering the effects of diffusion and degradation,the article hasestablished two mathematical expression groups concerning the history ofmaterials and mass loss process itself.By solving them,a general form ofthe function W(T,t)versus temperature and time,i.e.a bi-exponentialform,has been finally gotten.A corresponding fitting method has been devised and used to process andfit esperimental data.The results show that bi-exponential function has agood agreement with experiments.The mean square deviations are between10~(-3)-10~(-4).