Radial gates are used widely as a kind of control gates, worldwide. Although they are used widely, discharge estimation is not easy and few studies have been cited in the literature. In the present study, new relationships for estimation of discharge coefficients under free flow and submerged conditions are obtained. In these relationships, the discharge coefficient was related to the gate lip angle and the ratio of the water head to the gate opening height. By using these relationships, the accuracy of estimated discharge is increased. For this purpose, nonlinear multiple regression analysis and a set of laboratory data were used. Some statistical parameters such as maximum error, average error and standard error and three sets of field observed data were also used for evaluation of the relationships. One set of these data was collected in the present study and the others from previously published studies. The ranges of used observed discharge data were 0.030 to 0.320 and 0.010 to 0.180 m3s-1 in laboratory under free flow and submerge conditions, respectively. For the field observed data, these ranges were 0.11 to 239.6 and 0.05 to 209.02 under free flow and submerged conditions. Results showed that the relationships obtained could be appropriate for estimation of discharge coefficients in free flow and submerged radial gates. Relationships including gate lip angle, gate opening and upstream water depth are more reliable than the ones using only gate opening and upstream water depth. The average values of discharge estimation error could be about 2 and 1 percent under submerged and free flow conditions, respectively.