The study was carried out to evaluate the effect of feeding anhydrous ammonia treated rice straw (NH₃-RS) with two different levels of concentrate on daily gain, feed consumption and feed efficiency. Rice straw (RS) was treated with 30g NH₃/㎏ RS dry matter (DM). Fifteen heads of Korean native bulls were allotted to three groups (five animals/group) and fed the following diets: (1) RS+concentrate 2.0% (% of live body weight), (2) NH₃-RS + concentrate 1.5%, (3) NH₃-RS + concentrate 2.0% for 160 days, and RS and NH₃-RS were fed ad libitum. 1. There were no significant differences in daily gain among the bulls fed with RS + concentrate 2.0%, NH₃-AS + concentrate 1.5% and NH₃-RS + concentrate 2.0%. But bulls fed with NH₃-RS + concentrate 2.0% showed apparently increasing trends of daily gain compared with RS + concentrate 2.0% and NH₃-RS + concentrate 1.5% decreased slightly compared with NH₃-RS + concentrate 2.0%. 2. Rice straw consumption was significantly increased by anhydrous ammonia treatment but NH₃-RS consumption was not influenced by different concentrate level. 3. Concentrate was less required for unit body weight gain while rice straw was more required by feeding NH₃-RS.