〔Abstract〕 Objective To study the effects of different iodine nutritional status on methylation of B1 gene of carcinogen B1(BRAF) of rat thyroid sarcoma filter toxic bacteria(V-raf), and to explore whether iodine affects BRAF expression through methylation of BRAF gene. Methods 40 SPF female SD rats of different iodine nutritional status were randomly divided into 5 groups, namely low iodine group(LI), iodine suitable group(NI), and 5 times higher iodine group(5 HI), 10 times high iodine group(10 HI), 20 times high iodine group(20 HI), 8 rats in each group. The model was made by drinking deionized water with different iodine content and feeding low iodine feed. The urine of rats was collected for 24 h to measure urine iodine during the period, and rats were sacrificed after 3 months. Blood samples and thyroid tissues were collected, and the serum free triiodothyronine(FT3) and free thyroxine(FT4) and thyroid stimulating hormone(TSH) levels were determined by chemiluminescence. Bisulfite sequencing PCR(BSP) was used to determine the methylation of CPG island genes in the promoter region. Immunohistochemical technique was used to determine BRAF protein expression. Results ① Compared with NI group, the serum FT3 concentration in LI group and each HI group decreased, and the difference was statistically significant(P<0.05). ② Comparison of the methylation rate of BRAF in the five groups of rats was not statistically significant(P>0.05). ③ The expression of BRAF in thyroid tissues of LI group, NI group and HI group gradually increased, and the difference was statistically significant(P<0.05). Compared with the NI group, the expression of thyroid BRAF in LI group was lower, and the expression in each HI group was higher. Compared with 5 HI group, the expressions of thyroid BRAF in 10 HI and 20 HI group were higher, and the difference was statistically significant(P<0.05). Conclusion Iodine affects the expression of BRAF protein in rats' thyroid gland, but has no obvious effect on the methylation of BRAF gene, suggesting that iodine may affect the function of BRAF gene through other ways.