Photosensitized reaction on aliphatic polyesters, 3g4 and M(2)3g4, prepared from succinic acid and propanediol derivatives were investigated by spectrophotometry, GPC, and electron spin resonance (ESR) methods. From change of GPC curves for 3g4 polyester doped with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) as a photosensitizer, it was confirmed that not only degradation but also crosslinking reaction occurred for 3g4, M(2)3g4, and poly(lactic acid), by UV irradiation. By detailed analysis of ESR spectra, it was confirmed that several radical species are produced through photosensitized reaction; TMPD radical cation, ester radial anion of polyester, main chain alkyl radical at the center of diol component, and acyl-type radical. Although there was no direct evidence for producing the main chain scission radical for 3g4 and M(2)3g4, it was strongly suggested that the photosensitized degradation and the crosslinking reactions were originated from the ester radical anion that was produced by electron capturing.