Eight holstein bulls averaging 244.1 ㎏ in initial body weight were utilized in two treatments to study the effects of monensin on growth rate, feed efficiency, carcass characteristics and ruminal volatile fatty acids production in an outdoor drylot conditions. The treatment groups were fed ad libitum either 0 or 30 ppm monensin supplemented in concentrates, for 280-day feeding period. The cut rice straw as a only roughage was mined with the concentrates at the rate of 10%. During 112 days of the first stage, monensin improved daily gain 17%.(p$lt;.05), feed conversion 9%, and 11% and 10% in DCY and TDN per ㎏ gain, respectively. Throughout 280 days of tire entire period, monensin group showed 8% improvement in gain but had no improvement io feed conversion and DCP and TDN per ㎏ gain. Feed consumption tended to be higher as tire fattening stages progressed. Ruminal pH and total VFA concentration were unchanged by monensin, Molar percents of acetate and butyrate were reduced and that of propionate increased markedly with monensin. As a result, the acetate to propionate ratio was narrowed.