The objectives of this study were to estimate the protein degradability of extruded full fat soybean (ESB) by in situ (nylon bag) and in vitro enzymatic method and to develop an equation in order predict in situ degradability from in vitro values. In the study enzymatic technique; hydrolysis after 1 h (INV1) and after 24 h (INV24) by a purified protease extracted from Streptomyces griseus in a borate-phosphate buffer at pH 8 was used as in vitro method. Relationship between in situ effective protein degradability (INSE) and in vitro degradability after 1 and 24 hours incubations (INV1 and INV24) were determined. In situ protein degradability was measured at 0, 2, 4, 8, 16, 24, and 48 and at 72 h incubations in the rumen of 3 Holstein cows. In the study INSE, INV1 and INV24 were determined as 58.05, 20.24 and 41.46% respectively. Despite there were differences between in situ and in vitro protein degradability values, correlation coefficients between in situ and in vitro protein degradability of ESB were high and regression equations for estimation of in situ from in vitro were found significant. As conclusion in vitro enzymatic protein degradability (INV1 and INV24) can be used for estimation of in situ effective protein degradability of extruded full fat soybean.