Objective To investigate the effect of trichosanthin（TCS） on the first meiosis of mouse oocytes. Methods Totally 480 immature (GV-stage) oocytes were collected from the female mice (7-9 weeks old) at the 48th hour after they were received superovulation with 10IU of pregnant mare serum gonadotropin （PMSG）. They were cultured in the medium containing different concentrations of TCS. We observed the germinal vesicle breakdown (GVBD) percentage under the inverted microscope (anatomical lens). We detected the spindle formation and the chromosomes movement by immunofluorescence staining and then counted the number of the oocytes progressed to metaphaseⅠ(MⅠ) with laser scanning confocal microscope (LSCM). Oocytes were collected and observed to count the percentage of the first polar body (PB1) extrusion 16 hours after culture, served as the mark of the completion of the first meiosis. Results It was no significant deviation that the percentage of GVBD incidence and PB1 extrusion in the presence of TCS (30mg/L) compared with the control group; The lower concentration of TCS (30mg/L) interfered with the spindle assembly and the chromosomes alignment, which caused a significant decrease in the number of the oocytes progressed to MⅠand then extruded PB1; The higher concentrations of TCS (120-240mg/L) resulted in a significant decrease in the percentage of oocytes that underwent GVBD. Conclusion TCS interferes with the resumption of the meiosis, the spindle assembly and chromosomes alignment. And t