All-trans-retinoic acid (ATRA) can enhance iodine uptake capability of thyroid tumors, but the mechanisms remain poorly understood. The aim of the present study was to investigate the effects of ATRA on isotope susceptibility, proliferation and invasion of anaplastic thyroid carcinoma (ATC) and potential mechanisms. SW1736 cells were treated with 1 mu mol/l ATRA or 1% ethanol for 5 days. A cell line stably expressing beta-catenin-shRNA was established. An iodine uptake assay was performed using I-125. Proliferation and invasiveness were tested using MTT and Transwell assays, respectively. Western blotting was used to assess the expression of beta-catenin, glycogen synthase kinase-3 beta (GSK-3 beta), sodium/iodine symporter (NIS) and proteins involved in epithelial-mesenchymal transition. Cells pretreated with ATRA were injected subcutaneously into SCID mice. Mice were intraperitoneally injected with I-131 once on the first day of treatment, and tumor growth was then assessed. After 35 days of I-131 treatment, ATRA-pretreated tumor volume and weight were decreased compared with the I-131 alone group (163.32+/-19.57 vs. 332.06+/-21.37 mm(3); 0.35+/-0.14 vs. 0.67+/-0.23 g, both P<0.05). Similar results were observed in the beta-catenin shRNA-pretreated tumors. ATRA also increased the uptake of iodine by SW1736 cells (P<0.01), and similar results were observed in beta-catenin shRNA cells. ATRA treatment decreased the cell proliferation and invasion compared with control cells (all P<0.05), similar to beta-catenin shRNA. ATRA treatment decreased the expression of phosphorylated (p-) beta-catenin, p-GSK-3 beta, vimentin, and fibronectin, and increased the expression of NIS and E-cadherin, compared with the control. ATRA increased the iodine uptake and inhibited the proliferation and invasion of SW1736 cells, involving beta-catenin phosphorylation. In conclusion, ATRA could be used to improve the isotope sensitivity of ATC.