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利用微分扫描热分析（DSC）获得了聚对苯二甲酸乙二醇酯（PET）熔体等温结晶过程焓松弛速率曲线,并对结晶焓松弛速率曲线进行微分从而获得等温结晶焓松弛加速度曲线。文中通过定义相对结晶速率R和相对结晶加速度A,确定了相对结晶速率R和相对结晶加速度A与结晶焓松弛速率和结晶焓松弛加速度之间对应相差的一个常数值,然而其所对应的变化趋势却是相反的。根据结晶焓松弛速率和结晶焓松弛加速度的变化趋势,笔者认为PET熔体的等温结晶可以分为三个阶段,分别称之为阶段Ⅰ,阶段Ⅱ和阶段Ⅲ。其中结晶中间阶段（阶段Ⅱ）还可以分成两个＂亚阶段＂。利用结晶焓松弛速率和结晶焓松弛加速度的极值点确定了三个阶段及两个＂亚阶段＂的转化的临界时间点和相对结晶度。

Differential scanning calorimetry（DSC） method was used to obtained enthalpy relaxation rate of isothermal crystallization from poly（ethylene terephthalate）（PET） melt.The curve of enthalpy relaxation rate was differentiated and that of enthalpy relaxations accelerate was produced.The relative crystallization rate（R） and accelerate（A） were defined and connected to enthalpy relaxation rate dH/dt and accelerate d2H/dt2,respectively.Ratios of enthalpy relaxation rate and accelerate to relative crystallization rate and accelerate are constant.Changes tendencies of enthalpy relaxation rate and accelerate were opposite to those of relative crystallization rate and accelerate,respectively.The whole isothermal crystal process of polymeric melt can be divided to three crystallization stages based on the change tendency of enthalpy relaxation accelerate.The three crystallization stages were named stageⅠ,stageⅡ and stage Ⅲ.The stageⅡ was further divided to two sub-stages and named stageⅡ1 and stageⅡ2.The critical time points and relative crystallinity points,namely points of maximum relative crystallization accelerate,maximum relative crystallization rate and negative relative crystallization accelerate appeared under different crystallization temperature were made out from the extreme values in the curves of enthalpy relaxation rate and accelerate.