To suppress the environmental problems caused by livestock manure, we attempted to reduce the amount of nitrogen excreted in urine and feces by feeding low-crude protein (CP) diets to Japanese Black steers. The dietary CP contents were 12%-13% on a dry-matter basis in the control diet (CON, n = 4, initial body weight [BW] 268 kg) and 11% in the low-CP diet with amino acids (LCPAA, n = 4, initial BW 276 kg). The LCPAA diets were supplemented with rumen-protected lysine and rumen-protected methionine during the early and middle fattening stages. The nitrogen excretion in urine was significantly lower in the LCPAA-fed than CON-fed steers at the early (P < 0.01) and middle (P
< 0.05) fattening stages. In both cases, the plasma urea nitrogen concentrations were mostly lower in LCPAA- than CON-fed steers. The BW gain during the early fattening stage tended to be lower in LCPAA- than CON-fed steers (P < 0.10), but the dietary CP content affected neither the carcass weight nor the lean weight. The dietary CP content had no adverse effect on carcass traits. These results indicate that in Japanese Black steers, the 11% CP diet throughout the fattening period suppressed BW gain during the early period but reduced nitrogen excretion.