Abstract Studies were conducted to determine adequate rumen degradable nitrogen (RDN) levels for optimum microbial protein synthesis and digestibility of nutrients in adult Bandur sheep fed on finger millet straw (FMS) based diet. Thirty Bandur sheep were divided into five groups of six animals each using a balanced completely randomized design. Animals in all groups were fed FMS as a basal roughage and solvent-extracted soybean meal (SBM) as a nitrogen source. The sheep in group 1 (G1) were offered FMS for ad libitum intake. Animals in group 2, 3, 4 and 5 (G2, G3, G4 and G5) were supplemented with SBM to provide RDN level of 16.2, 20.7, 24.7 and 26.6g/kg digestible organic matter intake (DOMI) or 24.9, 31.8, 38.1 and 41.0g/kg digestible organic matter apparently digested in the rumen (DOMR) respectively, along with FMS. The daily total dry matter (DM) and organic matter (OM) intakes showed a linear increase (P<0.05) with increasing levels of RDN supplementation. Digestibility coefficients of DM, OM (linear: P<0.01), crude protein (CP), ether extract (EE) (quadratic: P<0.001), neutral detergent fiber (NDF) and acid detergent fiber (ADF) (quadratic: P<0.05) increased as the level of RDN supplementation increased from G1 to G5. The values for total purine derivatives (TPD) excretion (mmol/day and mmol/kgW0.75) (quadratic: P<0.05), the levels of microbial purine absorption and microbial nitrogen supply (MNSg/day, g/kgDOMI and g/kgDOMR) (quadratic: P<0.01) increased, while microbial nitrogen to total urinary nitrogen ratio (MN:UN) decreased (linear: P<0.01) with the increased RDN level, indicating the efficient conversion of RDN into microbial protein synthesis for G2 animals among the RDN supplemented groups. This study indicated that 16gRDN/kgDOMI or 25gRDN/kgDOMR may be adequate for optimum microbial protein synthesis and digestibility of nutrients, thereby improving plane of nutrition in sheep fed on finger millet straw based diet. The additional RDN supplied beyond certain limit (above 16gRDN/kgDOMI or 25gRDN/kgDOMR) may not be beneficial for improving the digestibility of diet especially on straw based diets.