Protein requirement of beef cattle are thought to be a function of many variables and not a specific figure for all conditions. Protein is one of the most expensive nutrients on a per baht basis and because of increased concerns that the excess nitrogen is washed down to rivers and streams causing pollution, it is imperative that we revaluate protein requirements of the rapidly gaining feedlot cattle. Two experiments on effect of protein level in beef steers were summarized. It was found that increased protein level was associated with excretion of nitrogen in feces and urine. The optimal protein level was set at 12%CP for growing and finishing beef cattle. From the experimental data, it can be concluded that in beef cattle the level of protein should be formulated to meet animal requirement and to reduce the loss of nitrogen to the environments.