Objective To study the invasion and proliferation in IEC-6 cells of Toxoplasma gondii RH strain tachyzoites in vitro. Methods T. gondii tachyzoites of RH strain were co-cultured with IEC-6 cells in vitro， the process of cell adhesion， invasion and proliferation by tachyzoites was observed consecutively with inverted microscope. At 5 min，10 min， 20 min， 30 min， 1， 2， 4， 6， 12， 24 and 48 h after co-culture， the tachyzoite invasion to IEC-6 and intra-cellular proliferation were observed with Giemsa-Wright′s staining， respectively. The invasive rate of tachyzoites to IEC-6 was counted. Results T. gondii tachyzoites invaded the IEC-6 cells 5 min after culture， thenceforth the invasive rate increased gradually. The invasive rate was about 55.0% at the first hour after culture with 1-5 tachyzoites in one cell. In the second hour after culture, the rate reached highest with 81.8 % and there were many pseudocysts emerging. At the same time， tachyzoites invaded the cell nucleus and proliferated in the nucleus. At the 4th hour after culture， the invasive rate began to decrease （80.8±9.2）%， the pseudocysts began to break and tachyzoites were released to cluster. The clustering tachyzoites increased significantly at the 6th hour. At the 12th hour the clustering tachyzoites decreased and most tachyzoites were free， the number of complete cells decreased obviously. There were only a few cells and pseudocysts left at the 24th hour， and a great quantity of free tachyzoites existed out of the IEC-6 cells. There were plenty of mobile tachyzoites while none of IEC-6 cells existed after 48 h culture. Conclusion IEC-6 cell may be the suitable target cell of Toxoplasma gondii tachyzoite. The tachyzoites can invade the IEC-6 cells quickly in vitro and proliferate in the plasma and nucleus with a reproductive cycle of about 6 to 12 hrs.