Objective To investigate the effects of icariin (Ica) and icaritin (Ict) on the proliferationin of breast cancer T47D cells in vitro and to explore the relationship between estrogen receptor (ER) and the mechanisms. Methods The ER positive T47D cells were treated with Ica and Ict at different concentration. The proliferation of T47D cells was measured by MTT method, and the cell cycle changes during the proliferation were further evaluated using flow cytometric assay. The effects of Ica and Ict on the expression of ERalpha and ERbeta in T47D cells were analyzed by Western blotting. Results Ica and Ict (10~(?9)10~(?6) mol/L) stimulated the proliferation of T47D cells, and the proliferation was inhibited by the estrogen antagonist ICI 182.780 (10~(?6) mol/L). Compared with the control group, Ica and Ict (10~(?7) mol/L) could increase the amount of T47D cells arrested in S phase, and the stimulation was inhibited by ICI 182.780. As well Ica and Ict (10~(?7) mol/L) could up-regulate the expression of ERalpha and ERbeta proteins (P < 0.01). Conclusion Ica and Ict stimulate the proliferation of T47D cells, and the effect may be performed through up-regulating the expression of ERalpha and ERbeta proteins.