To study the effect of crest shape on the performance of rectangular side weirs, one hundred and four side weir models were tested, eight of them were for sharp crest and ninety six for rounded crest. For each set of rounded crest models, the radius of crest was changed four times and for each radius, the height of the side weir was varied four times. A total of nine hundred thirty four tests were conducted on these models. For all models of different crest shapes, it was found that the average energy difference between two ends of the side weir is very small and less than one percent which can be ignored. The variation of discharge coefficient (CM) with upstream Froude number (Fr1) was found to be linear. The correlation between (CM) and ratio of upstream water depth to weir height ( h1/p ) was found to be linear. The variation of (CM) with the ratio of upstream head above crest to crest radius (h1/r) was also found linear. A simple power empirical expression was obtained for the combined effect of (Fr1) and (h1/p) on (CM) for free flow over sharp crest side weirs with high correlation coefficient. Three simple power empirical expressions were obtained for the variation of (CM) with (Fr1), (h1/p) and (h1/r) for the free flow over inside semicircular, outside semicircular and circular crest shapes with high correlation coefficients. From the hydraulic performance, it was found that small weir heights gave higher performance for all crest shapes. The highest performance of inside semicircular crest was found for crest radius (r) =3.15cm, the highest performance of outside semicircular crest was found for (r) =3.75cm, while, the highest performance of circular crest was found for (r) =2.5cm and (r/p) =0.25.