Anaplastic thyroid cancer (ATC) is a very aggressive malignancy that is resistant to various types of chemotherapy in humans. Most patients with late-stage ATC cannot undergo surgery and receive chemotherapy drugs. The present study investigated the influence of 1,25-dihydroxyvitamin D
3
(1,25(OH)
2
D
3
) pretreatment on adriamycin (ADM) chemotherapy efficacy in the 8305c and 8505c ATC cell lines. The apoptotic effects of ADM on ATC cells pretreated with 1,25(OH)
2
D
3
were evaluated. Cell viability was identified by using the Cell Counting Kit-8 assay. Apoptosis was assessed by flow cytometry and staining with Hoechst 33342. The expression of the apoptotic protein cleaved caspase-3 was tested with a colorimetric assay kit and by western blotting. Reactive oxygen species (ROS) generation was assessed with the antioxidant N-acetyl-L-cysteine (NAC) and the assay H
2
-DCFDA. In addition, ROS production could be reversed by NAC treatment. The present study demonstrated that 1,25(OH)
2
D
3
enhanced ADM-induced apoptosis in 8305c and 8505c cell lines. Furthermore, 1,25(OH)
2
D
3
improved the ADM-induced ROS production and expression of cleaved caspase-3. NAC treatment inhibited the expression of cleaved caspase-3 in ATC cells, and reduced apoptosis in cells that were pretreated with 1,25(OH)
2
D
3
and ADM. These results demonstrated that 1,25(OH)
2
D
3
may enhance ADM-induced apoptosis by increasing ROS generation in ATC cells.