Investigations have been carried out on gamma-ray irradiated thermoplastic polyurethane (PU). The change of molecular weight as a result of irradiation and the change of absorbance of samples eluted daily by methanol extraction were determined. GPC patterns, tensile strength, and residual amount of radicals based on chemiluminescence were also measured. PU chain-extended with 1,4-butanediol (BU) showed degradation, while PU without chain-extended with BU (linear PU) showed mainly crosslinking by gamma-ray irradiation. G values were 1.1 (degradation) and 0.2 (crosslinking). A linear relationship between the irradiation dose and the residual radical amount was found. There was no significant difference in molecular weight as determined immediately following irradiation with analysis 6 months after irradiation. The influence of the amount of residual radical on the changes in the characteristics of PU was not significant. The elution of PU oligomers (n = 1–10) was confirmed from the GPC pattern of methanol extract. The changes in tensile strength correlated well with the changes in molecular weight. Sequential daily methanol extract analysis showed that elution occurred semilogarithmically. Methanol extract at an early stage is more toxic due to abundant elution of lower molecular weight compound.