The ring-opening polymerization of ∈-caprolactone (∈-CL), initiated by carboxylic acids such as benzoic acid and chlorinated acetic acids under microwave irradiation, was investigated; with this method, no metal catalyst was necessary. The product was characterized as poly (∈-caprolactone) (PCL) by 1H NMR spectroscopy, Fourier transform infrared spectroscopy, ultraviolet spectroscopy, and gel permeation chromatography. The polymerization was significantly improved under microwave irradiation. The weight-average molecular weight (Mw) of PCL reached 44,800 g/mol, with a polydispersity index [weight-average molecular weight/number-average molecular weight (Mw/Mn)] of 1.6, when a mixture of ∈-CL and benzoic acid (25/1 molar ratio) was irradiated at 680W for 240min, whereas PCL with Mw 12,100 and Mw/Mn 4.2 was obtained from the same mixture by a conventional heating method at 210℃ for 240min. A degradation of the resultant PCL was observed during microwave polymerization with chlorinated acetic acids as initiators, and this induced a decrease in Mw of PCL. However, the degradation was hindered by benzoic acid at low concentrations.