Sixteen male Brown Swiss calve were utilize to study the effects of cereal source (barley and triticale) and monensin on fattening performance using a factorial experiment with four treatments. The number of replicates was four for each treatment and the experiment was last for 90 days. The experimental treatments were: 1) diet containing barley without monensin 2) diet containing barley and monensin 3) diet containing triticale without monensinand 4) diet containing triticale and monensin. The amount of monensin was set to be 30 mgkg-1 DM. Calf weighing was undertaken monthly, dry matter intake was measured daily. Sampling from rumen fluid was carried out using esophagous tube, and blood sampling was undertaken at the end of each month. The results indicated that average daily gain, feed conversion ratio, final weight and daily feed intake were not significantly affected by cereal source as well as monensin. Blood glucose, BUN and rumen PH were not significantly affected by the experimental treatments. The magnitude of rumen ammonia nitrogen had a non-significant decrease due to using monensin in the diet.