The thermal effect of microwave energy on ε-caprolactone (CL) monomer and polymerization reaction mixture of CL with stannous octoate [Sn(Oct)2] intimately depended on the power level of microwaves and the mass scale of materials. When 10g of CL was heated by microwaves at a power of 680W, its temperature increased rapidly from 20 to 355℃ within 5 min after which it was self-regulated to keep constant at 360℃. Heat generation was observed when a polymerization reaction mixture of 10g CL with 0.1% (mol/mol) Sn (Oct) 2 was irradiated at 680W. The exothermic peak was recorded at 2-5min and its temperature was over 300℃, with the maximum at 343℃. During this period the ring-opening polymerization of CL occurred quickly, resulting in poly(ε-caprolactone) with weight-average molecular weight (Mw) of 123,000g/mol and yield of 95%.