Abstract: OBJECTIVE To observe the radioactivity distribution and antitumor efficacy of ¹³¹I-bovine serum albumin (BSA)-polycaprolactone(PCL)-sorafenib in human anaplastic thyroid carcinoma cells 8305C-bearing nude mouse model. METHODS Prepared and identified BSA-PCL-sorafenib, ¹³¹I-BSA-PCL-sorafenib liposomes. Twelve nude mice were inoculated with human anaplastic thyroid carcinoma cells 8305C to establish model. After successful modeling, normal saline (control group), ¹³¹I, BSA-PCL-sorafenib, and ¹³¹I-BSA-PCL-sorafenib were injected into the tail vein. The radioactivity distribution in tumor-bearing nude mice at different times after injection was observed by SPECT/CT imaging, and the body weight and tumor mass and volume changes of tumor-bearing nude mice in each group were compared. RESULTS After incubation for 60 min, the fluorescence intensity in ¹³¹I-BSA-PCL-sorafenib group was higher than that in control group. After injection for 22 d, the weight and volume of tumor in ¹³¹I-BSA-PCL-sorafenib group was the smallest one in four groups. CONCLUSION ¹³¹I-BSA-PCL-sorafenib can effectively inhibit the tumor growth of 8350C induced-mouse tumor model, which gives a good suggestion for the treatment and prognosis evaluation of anaplastic thyroid carcinoma.