Abstract: The amino acid sub-model of Cornell net carbohydrate and protein system is one of the three published major model to predict ruminant duodenal amino acid flux at present. Previous studies indicated that the ruminal undegraded C protein fractions defined by CNCPS model of non-forage feedstuffs may be digestible in small intestine, which questioned the reliability of setting the digestibility of C protein fraction in small intestine to zero by the CNCPS model. Contents, dynamic degrading parameters in the rumen of each protein fractions, and CP digestibility of ruminal degrading residue at different sampling time point of 14 feedstuffs were measured in the present study. These data were used to estimate small intestine digestibility of each ruminal undegraded protein fraction by least square data fitting according a deduced equation. Reliable estimates were acquired when taking small intestine digestibility of ruminal undegraded C protein fraction was 100% as the restrictive condition. Results of the present study indicated that it was reasonable to set the small intestine digestibility of ruminal undegraded B1, B2, and B3 protein fractions as 100%, 100%, and 80% by the CNCPS model. Small intestine digestibility of ruminal undegraded C protein fraction of 8 feedstuffs, 4 of which were roughage, were high enough and can not be neglected, therefore it was reasonable to further investigate the reliability of setting the digestibility of C protein fraction to aero by the CNCPS model.