This study aimed at evaluating the effect of acidifying additives as formic and propionic acids on weight gain and on small intestinal macroscopic, microscopic and SEM morphology in ducks. A total of 30 Mulard ducklings were used in this study and they were randomly assigned to control (given standard commercial duck diets with no acidifiers added), 1% acidifiers added to the standard diet or 2% acidifiers added to the standard diet. After rearing the ducks for 52 days, their weight was recorded and then the length of the different parts of the small intestine was measured. Then, samples were taken for histological and scanning electron microscopical (SEM) examination of the small intestine. The present results found that addition of acidifying mixture to the diet significantly improved duck’s weight (P<0.05), also there were significant changes in length, histology and SEM characteristics of the small intestine between different duck groups. The observed effects were dose dependent, i.e. with more organic acids in the diet, the more the changes were noticeable. Our results indicate that, the addition of organic acids to duck’s diet increased small intestine’s length and increased the length of intestinal villi, possibly resulting in more surface area for digestion and absorption of the diet, and, therefore, greatly improved the final body weight of the study ducks.