Rate constants of the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) by microwave heating (MH) and flash conventional heating (FCH) have been measured. The reaction is catalyzed by stannous octanoate (Sn(Oct)2). With a microwave power level of 400, 500, 600, 700 and 800 W, the equilibrium temperature of the polymerization mixture increases from 165 to 180, 197, 213 and 228 °C. However, the rate constant does not continuously increase with the observed increase in temperature. Instead it abruptly increases at 180 °C (500 W), and the increase does not fit Arrhenius' law which, however, is followed by the polymerization under FCH conditions with an increase of the equilibrium temperature from 108 to 135, 151, 189 and 213 °C. The initiation time of the ROP by MH varies anomalistically with temperature and is not related to the rate constant, which is in contrast to that by FCH, which reduces exponentially from 127.5 to 36.40, 8.63, 1.22, and 0.579 min with the increase in polymerization temperature and rate constant. Based on the divergences it is reasonable to deduce that the acceleration of the ROP of ε-CL by microwaves is induced not only by thermal conditions (the thermal microwave effect) but also by the non-thermal microwave effect.