The mature oocytes of sweet fish Plecoglossus altivelis can maintain fertilization ability for a long time. In this study, we observed the ultrastructure′ changes of P. altivelis oocytes during post-ovulatory process using transmission electron microscope(TEM). In follicular phase, the outer layer of oocyte memebrane is zona radiate(ZR), which includes three-layers(ZR1, ZR2 and ZR3 from outside to inside, and the thickest layer is ZR1). Outside of ZR is jelly coat(JC). After ovulation, ZR1 disappears and ZR remains two layers (ZR2 and ZR3), and JC still exisst. In this stage, ZR2 is very thin, and ZR3 includes 5 light-bands alternate with 5 dark-bands. ZR3 is completely opaque, and in the vertical of oolemma showed alternating "light region" and "dark region". At 48 h after ovulation, JC disappears, ZR is thinner and pore canals(PC) appears in ZR3. At 60 h, the number of PC increases and diameter enlarges. To 96 h, the length of PC increases to 1/2 of the ZR3. 104 h later, with the disintegration of ZR2, PC continues to elongate and pass through ZR3, and then the mature oocyte completely apoptosis.