Objectives：To investigate the effects of tamoxifen（TAM） on the growth of estrogen receptor（ER） negative thyroid carcinomas and the mechanisms involved. Methods：In vitro experiments, papillary thyroid carcinoma cells（IHH-4） and follicular thyroid carcinoma cells（FTC-133） were treated with tamoxifen in different doses and different lengths of time, using the dimethylthiazol-dipheltetrazolium bromide（MTT） method to observe the cell proliferation. Cell cycle and apoptosis were analysed with flow cytometric DNA studies. In vivo experiments the papillary thyroid carcinoma cells were injected into the nude mice subcutaneously, and the mice had been feed with TAM［5 mg／（kg·d）］ daily since the second week. The tumor volumes were calculated weekly. Finally the mice were sacrificed and the tumors were weighed separately. Results：In vitro，TAM inhibited the proliferation of the IHH-4 and FTC-133 cell lines in dose-dependent and time-dependent pattern. TAM inhibited the growth of the IHH-4 and FTC-133 cell lines by blocking the cell cycle in G1 phase. Immunohistochemistry and RT-PCR studies showed the ER negative in both cell lines. In vitro，TAM also inhibited the growth of IHH-4 xenografts in nude mice. Conclusions：TAM inhibits the growth of thyroid carcinoma in vitro and in vivo. TAM acts independent of estrogen receptor and could be a potential choice for the treatment of the aggressive thyroid carcinomas clinically.