There is increasing interest in the Cornell Net Carbohydrate and Protein System (CNCPS) for rationing cattle. Crude and digestible protein determinations do not completely account for the dynamics of ruminal fermentation and potential loss of nitrogen as ammonia (Sniffen et al 1992). When measured data on the protein and carbohydrate contents of feeds were used, the predictions of the performance of dairy, beef and dual-purpose cattle were more accurate than when tabular values were used (Fox et al 2004 ). To obtain the best predictions, the CNCPS model requires an accurate description of the carbohydrate and protein fractions and the unique rates of digestion and passage of individual feed fractions that are being fed. This information can be used as a basis for amino acids absorption. The CNCPS use in Iran is limited because the inputs data is not determined. The present work was carried out to determine nitrogen fractions of some feeds are using in ruminant nutrition.