Objective:To investigate the influence of 32P on the biodegradation and radioactivity release rate of 32P-chromic phosphate-poly (L-lactide)(32P-CP-PLLA) seed. Methods:32P-CP-PLLA seeds with increased radioactivity and specific radioactivity were prepared(noted as S1-5) and their biodegradation,weight loss,radioactivity release rate and ultrastructure changes were investigated in different releasing medium such as fetal bovine serum,saline and dextran(noted as X,Y,Z),and after intratumor implantion. Results:32P-CP-PLLA seeds with increasing radioactivity were demonstrated increased radiation release rate at same time points,especially since day 32 and the increase seems corresponding with the change of the radioactivity of seed. And the increase of biodegradational weight loss matched with the increase of radiation release rate after intratumor implantation. The radiation release rate was much faster in the fetal bovine serum(sterile environment) than that in other medium and in vivo,and bacteria were found in the seeds by electron microscope. Conclusion: 32P may serve as one important modulator of the biodegradation and radiation release rate of 32P-CP-PLLA seed,and bacteria with possible secreting matters may also influence the biodegradation of PLLA.