The objective of this study was to evaluate the effect of using white kabesak leaves in the concentrate on biomass production, apparent and true degradability, and undegraded protein of rations In Vitro. This study used the In Vitro gas production method, designed using a group randomized experimental design. The proportion of grass forage and concentrate in the diet was 60:40. The treatment of the use of white kabesak leaves is R0: without white kabesak leaves (0%), R1: 10%, R2 20%, R3 30% and R4 40%. The results showed that the treatment had a very significant effect (P<0.01) on microbial biomass production, Apparent and True Degradability, and a considerable impact (P<0.05) on undegraded dietary protein rations. Biomass production decreased from 77.30 mg/500 mg DM in R0 to 43.6 mg/500 mg DM in R4. Apparent degradability from 287.25 mg/500 mg DM in R0 to 268.42 mg/500 mg DM in R4, and True degradability from 364.87 mg/500 mg DM in R0 to 312 mg/500 mg DM in R4. Undegraded dietary protein increased from 8.65% in R0 to 13.93% in R3. It was concluded that the use of white kabesak leaves in concentrates In Vitro produced a positive effect by decreasing feed protein degradation and increasing undegraded protein, making it more available post-rumen. The use of 20% white kabesak leaves in concentrate is the ideal level for In Vitro nutrient acceptance.