This paper presents the results of analytical and experimental investigation on subcritical flow in sudden expansion in rectangular rigid-bed channels with expansion ratios of 1.5 to 3.0. In addition, data collected by other investigators are also used. Theoretically the shape of separating stream line is predicted on the assumption that the sum of friction and expansion losses in expansion is minimum. Using the data mentioned above, velocity distribution in the vertical, distribution of mean velocity across the width, distribution of shear on the bed, lengths of the standing eddy, shape of separating streamline, and variation of energy loss coefficient have been studied.