In vitro crude protein digestibility (IVCPD) followed by in vivo studies were conducted to determine the influence of mono-component cysteine protease enzyme supplementation on digestibility of commonly used vegetable protein sources in layer chicken feed viz., soybean meal, sunflower meal, rapeseed meal and de-oiled groundnut cake. The in vitro crude protein digestibility (IVCPD) was done by two-step (Pepsin- Pancreatin) in vitro assay. In this study, control group had only test ingredients, whereas in treatment groups test ingredients were supplemented with cysteine protease (@ 150 gm/ ton). Cysteine protease supplementation in feed ingredients increased (P<0.05) the IVCPD in sunflower meal (73.38 vs. 67.17 %) and rapeseed meal (70.73 vs 66.03 %). IVCPD of soybean meal and de-oiled groundnut cake was not influenced. Following the in vitro study, an in vivo protein digestibility experiment was conducted in commercial layer cockerels (14 weeks of age). Four dietary treatments were formulated viz., 1) Control diet without enzyme; 2) Control diet plus NSP -ase (@500 g/ ton); 3) Control diet plus cysteine protease @ 150 gm/ ton); 4) Control diet plus both NSP -ase and cysteine protease enzyme. Sixteen cockerels were used in the metabolic trial with 4 birds in each dietary treatment. There was no significant difference in dry matter and crude protein digestibility due to cysteine protease and NSP -ase supplementation. However, the crude protein digestibility increased non- significantly to the tune of 2.11 and 5.11 % as compared to control group when cysteine protease alone or cysteine protease plus NSP-ase, respectively From this study, it can be concluded that cysteine protease enzyme supplementation increases protein digestibility in chicken.