The purpose of this study was to examine effect of mixed acidifier supplementation as feed additive on small intestinal morphology in laying duck. The acidifier used was in 2 forms, namely formic and fumaric acids and mixed at ratio of 1:1. This research method used in vivo experiment by using Completely Randomized Design (CRD). One hundred and fifty female ducks aged 50 weeks were randomly distributed into 5 treatments and 5 replications. The treatments consisted of T0-= basal feed/control. T0+= basal feed with addition of Bacitracin antibiotic 0.1%. T1= basal feed + 0.1% mixture of formic and fumaric acids at ratio of 1:1, T2= basal feed + 0.2% mixture of formic and fumaric acid at rasio 1:1, and T3= basal feed + 0.3% mixture of formic and fumaric acids at ratio of 1:1. The variables observed were intestinal morphology of ileum consisting of villus number, villus height, crypt depth, basal width, apical width, and villus surface area. The results showed that there were significant effects (P>0.05) in the villus height and apical width and highly significant effects (P>0.01) in the total villus number. On the other hand, treatments showed no difference effect (P<0.05) on crypt depth, basal width and surface area. In conclusion, the acidifier treatment improves intestinal morphology, especially when formic and fumaric acid mixture were supplemented at 0.1%.