In situ procedure is a direct method of measuring the rumen degradation kinetic of a feed nutrient. Data obtained by this technique are generally analysed using an exponential curve (Ørskov and McDonald, 1979). However, very low attention has been paid to choice of mathematical model to fit the curves and goodness-of-fit of the model. Lopez et al. (1999) pointed out that the disappearance of some feed components, particularly structural carbohydrates, exhibits a larger variety of forms than for crude protein (CP). In the present study, two different mathematical models of a straight line or a negative exponential (France et al., 1990; and Ørskov and McDonald, 1979) were selected to evaluate in situ degradation kinetics of protein fractions including true protein (TP), neutral-detergent insoluble protein (NDIP) and acid-detergent insoluble protein (ADIP) of various oilseed meals (cottonseed meal (CSM), soyabean meal (SM) and rapeseed meal (RM)).