Abstract: In vitro gas production was measured to investigate associative effects of untreated and chemically treated Rice Straw (RS). The RS was treated with NaOH, urea or lime and evaluated the nutritive value of treated rice straw using in vitro gas production. Cumulative gas production was recorded at 2, 4, 6, 8, 12, 36, 48, 72 and 96 h of incubation and the analyzed to describe the kinetics of gas production. Total nucleic acid was extracted from ruminal content in vial of each treatment and contribution of fungal population was estimated by using PCR-DGGE technique. It was found that the treatment with NaOH, urea and lime increased (p<0.05) gas production at 24 h, potential GP and rate constant of GP were highest for NaOH, follow by 2% urea+2% lime, 3% urea, 3% lime, 3.5% urea+3.5% lime and lowest for 5% urea treated rice straw (p<0.05). Ammonia nitrogen was increased belong to the increase of urea level treating rice straw. It was higher in 5% urea treatment and 3% urea-lime treatment and 2% urea-lime treatment. Total VFA and acetate and propionate concentrations were higher for 3% urea and 2% urea-lime as compared with other treatments (p<0.05). All treated rice straw shown similar in diversity of fungi except 2% sodium treated rice straw. Other treated rice straw was found similar in the diversity of fungi with 6 bands per each lane. The results from this study suggest that 2% urea plus lime treated rice straw can use as good roughage for ruminants to improve rumen fermentation, digestibility and low cost and treatments rice straw were shift the number species of rumen fungi.