Abstract: Effect of different organic nitrogen sources on inosine biosysnthesis was investigated through the analysis of proteins,nucleic acids and biomass.Results show that domestic yeast extract A can be the ideal nitrogen source for inosine fermentation.In addition,with its partial solubility,the carbon chain of amino acids hydrolysed from domestic yeast A believed to be the precursors to glycolysis pathway and tricarboxylic acid cycle.Usage of domestic extract A as nitrogen source results in the decrease of the respiration quotient(RQ) value from 1.0 to 0.6 during the inosine production phase and inosine production is elevated correspondingly.The yield coefficient of inosine to glucose increases from 15% to 29%.