Abstract: Background and Objective: Protection of proteins is essential for productive ruminants, this study aimed to obtain optimal level of gambier leaf residue (GLR) that can protect the protein of cattle feed supplement (CFS) from rumen microbial degradation. Materials and Methods: Gambier (Uncaria gambir Roxb.) leaf residue containing 9.96% condensed tannin was added to CFS containing 29% crude protein with a mixture of brown sugar, soybean meal, coconut cake, tapioca, urea and minerals at level 0 (control), 2.5, 5.0 and 7.5%. Condensed tannin content in CFS: 0, 0.68, 1.17 and 1.60% then which were tested in rumen digestion in vitro. Results: The addition GLR in CFS can protected protein in CFS with decreased ruminal protein degradation 16.84% compare control. Rumen undegraded dietary protein (RUDP), TCA-soluble N and peptide N was increase by addition GLR in average 42.38% compared control. The optimum level of GLR in CFS was obtained 4.88% with the minimum rate of protein degradation in 1.05% h–1. Conclusion: The addition of GLR in CFS can protected protein from microbial degradation in rumen with optimum level of 4.88%.