Background: Diarrheal diseases is one of the major causes of mortality and the most common health problems in developing countries and have an important role in malnutrition of children. Due to the effects of diarrhea and malnutrition on each other, improving diarrhea treatment programs are important for children's health. The aim of this study was to evaluate the effect of vitamin A in improvement of watery diarrhea in 1-5 years old children. Methods: This study was performed on 227 children aged 1-5 years affected by acute watery diarrhea admitted to ShahidBeheshti hospital in 2014. Patients were divided into two groups, vitamin A group, and control group. Duration of diarrhea, the average of diarrheal episodes, and length of hospitalization were recorded. Data was analyzed by the use of Chi-squared and t-tests. Findings: The study included 227 patients in two groups of 116 patients (intervention group) and 111 patients (control group). It was found that the average duration of diarrhea in the group receiving vitamin A, was significantly lower (3.24 +/- 1.64 days in the intervention group and 4.95 +/- 1.73 days in the control group, P < 0.001) as well as Diarrhea frequency (5.78 +/- 2.00 vs. 7.77 +/- 1.58 respectively, p<0.001) and length of hospitalization (4.03 +/- 1.63 days in the intervention group and 5.36 +/- 1.70 days in the control group, P < 0.001). Conclusion: Intramuscular vitamin A prescription in the treatment of diarrheal diseases in children reduces the duration and severity of diarrhea.