Twenty multifarious Holstein cows were used in randomized complete block to investigate the effect of increasing level of canola meal as the alternative plant protein supplement on milk production and composition. In the experiment cottonseed meal was substituted with canola meal. From rations were formulated, diets I through 4 contained 40% forage: 60% concentrate and consisted of 0, 8, 14, 25% canola meal, respectively. The amount of glucosinolate in canola meal was 2.69 micromole pergramme. Also diets in four treatments were isonitrogenous and iso caloric and fed as total mixed ration for eight week. Cows in treatment with high level of inclusion of canola meal, had highest amount of serum cholesterol and lowest level of thyroxin hormone, and the differences between treatment were significant (p<0.05). Other blood metabolites, Ca, p, glucose, albumin, total protein and triiodotyronin were similar. No differences were found among treatments for milk yield and component but diet with high level of inclusion of canola meal tended to produce more 4 and 3.2% fat corrected milk. Ruminal and fecal pH, were not different, but tended to increase in diets I throuth 4. Digestibility of dry matter, crude protein, neutral fiber, non carbohydrate, and ether extract were similar among diets, but degradability of dry matter and crude protein of canola meal higher cotton seed meal.