A mechanistic model of nutrient digestion in the rumen of dairy cows has been developed to predict rates of nutrient digestion and passage as well as rumen NDF pool sizes at fixed rates of dry matter intake. Different feed fractions according to chemical and physical characteristics are considered. There are 18 rumen state variables representing carbohydrates, nitrogen and microbes in the model. The model is mainly sensitive to the parameters defining rate constants of passage of small particles and digestion rate constant of NDF. For comparative analysis the model has been tested with diets differing widely in intake level and composition. NDF passage and digestibility, NDF pool, total and non-ammonia nitrogen outflow from rumen are well predicted for the whole range of diets used. For some diets, especially those containing maize as a source of starch the predictions of OM and starch digestibility and partition of NAN outflow between dietary and microbial N outflow are not satisfactory.