: Transitions are hydraulic structures which are used in the case of changing cross sectional dimensions in a relatively short distance. They include contractions and expansions. Contractions are used for the reduction, but expansions are for the broading of the channel section. Certain flow discharge measurement or control gates in channels may be called underflow gates. Common examples are the sluice and radial gates. In literatures, many researches related to the sluice gates, have done without contractions at the upstream reach of the gate. In this research, the main aim was the usage of a contraction for reducing undesirable vortices at the upstream reach of the sluice gates, reducing the disturbances and standing waves at the downstream reach of the gate and also reducing the energy loss of the flow through the gate. Experiments are completed in a channel with 0.405m width and 9.5m long. The channel upstream width was 0.405m and at the downstream reach, channels of 0.2, 0.24 and 0.3m were used. In various designs of transitions, a straight length, paralleling to the flow direction with the same width as the gate and inclined length for transition have been used. The relative lengths of the straight one to the width of the gate were 0, 0.8 and 1.4. Angle of the inclined one with the flow direction was 30, 45 and 90 degrees. Totally, 9 experimental models for each downstream width of the channel i.e., 27 models and 1152 runs on the models with free and submerged flow at the downstream reach have been done. To estimate free and submerge discharge, 5 semi-theoretical models using the Energy, Momentum, Continuity equations and also equations of Froude numbers of flow T the upstream reach of the transition and gate cross section are presented. Equations have given without contraction coefficients. Comparison a few data of this research with some experimental data of the other investigators, which have done in the same experimental conditions, proved the accuracy of the experiments. The results of this research showed that the straight length of transition, paralleling to the flow direction, is more effective in reducing disturbances and standing waves at the downstream reach of the gate, but inclined part of the transition reduces the intensity of vortices, appearing at the upstream reach of the gate. Transitions with only inclined angle give the best result in reducing the flow energy loss. Results of this investigation gave the following recommendations: First, using a transition containing an inclined wall with a contraction angle, equal or less than 30 degrees and a straight wall, paralleling to the flow direction with a length not less than the gate width; Second, applying the Froude number at the upstream reach of the gate to predict the flow discharge under the sluice gate.