This research was carried out to determine the effect of formaldehyde treatment on in-situ and effective dry matter degradabilitties (DMD), organic matter degradabilities (OMD) and crude protein degradabilities.(COD) of soybean meal (SBM), hazelnut kernel oil meal (HKOM), sunflower oil meal (SM) and cotton seed meal (CSM). Effective dry matter degradabilities of untreated HKOM, SBM, SM, FFS and CSM and of those treated with formaldehyde were found as (k=0.05) 66.05,67.35, 56.60, 53.25, 56.50 % and 39.20, 45.90, 37.25, 40.35 and 40.90 %, respectively. Effective organic matter degradabilities for the same feeds as 66.80, 69.45, 56.30, 54.00, and 58.55 %; 40.35, 43.80, 36.85, 40.40 and 41.00 %, respectively. Effective protein degradabilities (k=0.05) were found as 66.70, 65.80, 68.20, 49.05 and 65.40 %; 31.50, 36.40, 33.95, 34.35 and 34.35 % for untreated HKOM, SBM, SM, Full-Fat Soya bean (FFS) and for formaldehyde treated HKOM, SBM, SM and FFS, respectively. The differences among protein feeds in terms of degradabilities and degradability parameters (b and a+b) were found significant (P<0.01). Formaldehyde treatment decreased the rumen degradabilities of protein sources (P<0.01). The degradabilities of feeds high in fiber and fat were found lower than the other feeds. And also it was concluded that formaldehyde treatment was a useful method in order to decrease the rumen degradabilities and to increase digestibilities of the feeds.