A novel method to determine a glass transition temperature (T-g) of spin-labeled polymer applying the continuous-wave electron spin resonance (CW-ESR) has been developed. Poly(cyclohexyl acrylate) (PCHA), polystyrene (PS), poly(methyl acrylate) (PMA), poly(propylene glycol) (PPG), and poly(ethylene glycol) (PEG) were labeled by attachment of nitroxide radicals; the temperature-dependent microwave power saturation measurements were carried out. The saturation factor, S, determined from the microwave power saturation measurement, had different temperature dependence above and below the T-g of the spin-labeled polymer because of the different temperature dependence of the spin-lattice relaxation time. The inflection point in the temperature dependence of S was defined as a T-g, (ESR). The T-g, ESR was in good agreement with the T-g, (DSC) determined by differential scanning calorimetry (DSC) for the PCHA, PS, PMA, and PPG. The T-g, (ESR) of the PCHA labeled at the chain end was about 3 K lower than that of the one labeled at the midchain segments, which indicated that the local T-g around chain ends is slightly depressed. Furthermore, the T-g, (ESR) of highly crystallized PEG was determined to 221 K even though the DSC was unable to detect the glass transition. This value agreed well with the T-g of PEG determined by thermally stimulated current.