This study was conducted to determine the effects of rumen-protected methionine (RPM) supplementation on growth performance, nitrogen (N) balance, carcass traits, and meat quality of lambs fed diets containing buckwheat straw. Twenty male Tan lambs from Ningxia were housed in individual pens and were randomly assigned to five dietary treatments that were supplemented with RPM at levels of 0 (control), 1.5 (T1), 3.0 (T2), 4.5 (T3), and 6.0 g d^-1 (T4). Dry matter intake was higher in T4 than control (P < 0.05). The feed:gain ratio generally reduced with increasing dietary RPM, but the lowest ratio was found in T3 (P < 0.05). Dry matter digestibility and N retention were highest in T3 (P < 0.05) while N excretion was the lowest. Carcass fat weight and grid reference tissue depth increased while drip loss decreased by supplemental RPM, and significant differences were found between T4 and control (P < 0.05). However, pH, color, cooking loss, and shear force were not affected. The results from this study demonstrated that dietary RPM supplementation may be able to improve the feed efficiency, N utilization, and meat quality of lambs; however, the optimal dose of RPM needs to be explored further.