An experiment was conducted to determine the effects of dietary supplementation of prebiotic, betaine and their combination in duck diets on growth performance, nutrient digestibility, carcass criteria and cecum microbial population of ducks under hot environmental conditions. Treatment groups were fed on a control diet, the control diet supplemented with prebiotic as Bio mannanoligosaccharides (Bio MOS, 1 g/kg), the control diet supplemented with Betaine (1.5 g/kg) and the control diet supplemented with a combination of Bio MOS (1 g/kg) and Betaine (1.5 g/kg). A total of 80 Mollar ducks (average weight = 620.91 g, 20 day of age) were randomly assigned into 4 equal treatment. Each treatment was sub-divided into 5 replicates pens (4 ducks per pen) for 35 day from 20th of April to 25th of May 2017 in open house. Supplementation of Bio MOS and Betaine, separately as well as combined, significantly improved growth performance and feed conversion ratio. Digestibility of crude protein and crude fiber had significantly increased by the three additions. Total count of E.coli bacteria had decreased significantly and the total count of lactobacillus bacteria had increased significantly in prebiotic and combination treatments. Carcass weight, dressing percentage, intestinal weight and live body weight had improved significantly by the three treatments comparison with control one, however, liver, heart, spleen, gizzard, cecum and head weight didn't affect by the additions.