We conducted a study to clarify the mechanism of crosslinking and degradation of thermoplastic polyurethane (PU) when irradiated with gamma-rays. Changes in molecular weights as a result of gamma-ray irradiation, the UV absorption, the amount of PUs with amino groups, and the elution of oligomers to an organic solvent were determined to estimate characteristic changes in PU caused by irradiation. The amount of PUs with primary amino groups as a degradated PU products at p,p′-methylenediphenyl diisocyanate–1,4-butanediol (MDI–BU) hard segments increased linearly with increasing irradiation level. The amount of this products by gamma-ray irradiation was approximately 10–20% of the calculated amount, indicating that predominant degradation occurred at polytetramethyleneglycol (PTMG) soft segments (about 80–90% of the degradation). Characteristic differences were seen in crosslinking between non-chain-extended thermoplastic PUs based on the molecular weights of PTMG, (Mw of PTMG = 640–2800). The crosslinking ratio is linearly proportional to the molecular weights of PTMG, indicating that crosslinking at PTMG soft segments was major (the ratio of crosslinking at PTMG was more than 90%). Methanol extract of PU indicated elution of PU oligomers ranging from 13 to 1 as a polymerization degree.