The effect of diet type and fat addition on cellulose degradability was estimated on four rams fitted with rumen cannulas in a 4x4 Latin square design experiment. The animals were fed diets composed of 100% meadow hay (1^st trial) or 40% meadow hay and 60% concentrate (2^nd trial). Diets were supplemented with 0, 4, 8 or 10% rape seed oil, linseed oil or tallow. There were three experiments in each trial. Effective degradability of cellulose may vary among different fat sources and type of diet. Adding rape seed oil decreased effective degradability and when the animals were given meadow hay (1^st trial), differences were significant (P≤0.05). A decrease was observed in effective degradability when linseed oil was added to meadow hay (1^st trial), but the differences were not significant, whereas linseed oil added to concentrate-hay rations (2^nd trial) significantly (P≤0.05) increased effective degradability. Adding tallow had no effect on effective cellulose degradability in the rumen of sheep.