Polydopamine nanoparticles (PDAPs) with an average particle size of about 180nm were irradiated with a Co-60 source. The morphology, thermal stability and free radical signal of gamma-irradiated PADPs were characterized. The results suggested that a 200 kGy irradiation dose has little effect on PDAPs. As the gamma-irradiation dose increases to 400 kGy, PDAPs degraded significantly, morphology of PDAPs was obviously deformed, thermal stability and EPR intensity was significantly decreased. Furthermore, a degradation mechanism of PDAPs under gamma-irradiation was explored.