In this experiment, the effect of prebiotic Bio-Mos® (MOS) and types of feed formulation (total and digestible amino acids) were evaluated on performance and the small intestine microflora in laying hens for a duration of 10 wks. A total of 168 Hy line W-36 laying hens, with an initial age of 73 wks, were randomly allocated to 4 dietary treatments, 7 replicates and 6 birds in each replicate. The study was conducted in a completely randomized design with 2 × 2 factorial arrangement of treatments including 2 preboitic levels (0, 0.5 kg/ton of diet) and 2 types of AA feed formulation (total and digestible). Egg production (EP), egg weight (EW), egg mass (EM), average daily feed intake (ADFI) and feed conversion ratio (FCR) were not affected significantly by MOS, types of AA feed formulation and their interaction. Egg quality parameters including, specific gravity (SG), shell thickness (ST), shell weight (SW), haugh unit (HU) and yolk color (YC) did not affected by P-BM and types of AA feed formulation (P>0.05). No significant effects were observed on ileum Lactobacillus count by MOS, types of AA feed formulation and their interaction (P>0.05). There were no significant differences in the rate and gas production volume of experimental treatments (P>0.05). In conclusion, feed additive used in this study did not significantly affect the laying hens performance; intestinal Lactobacillus count, cecum gas production and microbial population. پرونده مقاله