Objective: To investigate the joint effects of excessive fluoride and iodine in drinking water on school-age children's dental fluorosis in the countryside of Tianjin. Methods: Based on the screening results of fluorine and iodine contents in drinking water in rural areas of Tianjin, three towns in Jinghai County and Wuqing District of Tianjin were selected as high fluoride and high iodine group, high fluoride and suitable iodine group, suitable fluoride and suitable iodine (control) group. Children aged 8-10 were selected as survey subjects. Morning urine was collected and dental fluorosis was examined. The concentration of urinary iodine was determined using arsenic cerium catalytic spectrophotometry (WS/T 107-2006), and urinary fluoride was detected using fluoride ion selective electrode method(WS/T 89-1996). Dean method was used to check the prevalence of dental fluorosis. Results: The urinary iodine median was 721.7, 788.3, 293.5 μg/L, respectively, and urinary fluorosis median was 2.48, 2.70, 1.59 mg/L, respectively. Urinary iodine and fluorosis were the highest in high fluoride and high iodine group, followed by high fluoride and suitable iodine group and control group(x2 = 35.10, 17.01, all P < 0.05). There was a significant difference of prevalence of dental fluorosis among the three groups(x2 = 15.39, P<0.05). The prevalence of dental fluorosis was the highest in high fluoride and suitable iodine group (85.3%, 29/34), followed by high fluoride and high iodine group(66.7%, 14/21) and control group(48.8%,21/43). Multiple Logistic regression analysis showed that the prevalence of dental fluorosis was adversely associated with fluorine concentration in drinking water[OR =7.273, 95% confidence limits(C/):1.676-31.562, x2 = 7.020, P < 0.05]. Conclusion: Drinking water with high fluoride and exposure of high fluoride combined with high iodine has a definite impact on children' s dental fluorosis.