Background: Zinc is essential for nucleic acid metabolism, protein synthesis and tissue healing. Zinc deficiency has detrimental effects on child health and increases mortality rate of pneumonia and gastroenteritis. By improving absorptive function of villous cells and by reducing pathogenic enterobacteriaceae in lumen, Zinc improves diarrhea outcome in children. Previous studies in developing countries are available regarding zinc efficacy in acute and persistent diarrhea. We tried effectiveness of zinc treatment in acute diarrhea of children in Lorestan province.Materials and methods: A triple blinded randomized controlled trial was planned over test and control groups. Each group contained 36 children aged 12 to 60 months with acute watery diarrhea whom weight for height was below -2SD under median according to Waterlow's classification. Test group took 1mg/kg/day zinc (in the form of 1% zinc sulfate solution). Then, diarrhea duration, daily times of bowel movement and weight gain were recorded. Data were analyzed by SPSSVersion13.Results: T-test showed that daily diarrhea in test and control group was 3.3±0.99 times and 4.9±1.1 times respectively. Duration of diarrhea in them was 4.9±3.2 days and 7.8±3.9 days. Weight gain was 223.05±110.8 grams and 95.8±106.4 grams. The results were significantly different between two groups at p<0.05. No child in zinc group suffered persistent diarrhea or weight loss.Conclusion: Zinc sulfate could be a part of acute gastroenteritis treatment plan in children besides other nutritional and hygienic orders especially in malnourished children in developing countries.