The effect of phosphorus supplementation to ammoniated rice straw was studied. The in vitro experiment was carried out following the first stage of Tilley and Terry method. The treatments consisting of four diets were A= 50% ammoniated rice straw + 50% concentrate (control), B= A + 0.2% phosphorus (P) supplement, C= A + 0.4% phosphorus (P) supplement, and D= A + 0.6% phosphorus (P) supplement of dry matter. Completely randomized design was used as the experimental design with differences among treatment means were examined using Duncan’s multiple range test. Variables measured were total bacterial and cellulolytic bacterial population, cellulolytic enzyme activity, ammonia (NH3) and volatile fatty acid (VFA) concentrations, as fermentability indicators and synthesized microbial protein, as well as degradability indicators including dry matter (DM), organic matter (OM), neutral detergent fibre (NDF), acid detergent fibre (ADF) and cellulose. The results indicated that fermentability and degradability of diets consisting ammoniated rice straw with P supplementation were significantly higher than the control diet (P<0.05). It is showed that P supplementation is important for rumen fermentation and growth of rumen microbes. Overall supplementation of phosphorus at 0.4% of dry matter to ammoniated rice straw shown best results in terms of rumen fermentation, microbial protein synthesis and in vitro degradability. پرونده مقاله