The N-acetyl-l-methionine (NALM) is one of methionine (Met) derivatives produced via protection of l-Met α-amino group with an N-acetyl group and has been shown to be bioavailable and capable of replacing the dietary requirement for Met in animals and humans. The current experiment was conducted to test a hypothesis that lactating dairy cows fed with NALM would increase milk production by increasing N and energy utilization efficiencies in a dose dependent manner. Eight multiparous Holstein cows that were mid lactation (124 ± 13 d-in-milk) with similar milk production were used in a 4 × 4 Latin square design for 84 d. A developmental NALM product from CJ CheilJedang (Seoul, South Korea) was used as the supplemental source of rumen-protected Met in the present study. Four dietary treatments included 0 g (control), 15 g, 30 g, and 45 g/d/cow of NALM supplementation. Supplementing NALM sizably increased dry matter intake (linear effect; P < 0.01), while milk yield tended to increase quadratically (P = 0.07). A linear decrease in milk fat concentration was seen due to NALM treatments relative to the control (P = 0.02). However, milk fat yield was similar across treatments. A trend toward an increase in milk protein yield was observed between the control and the 45 g NALM (1.18 vs. 1.21 kg/d; P = 0.10). There were no differences in energy-corrected or 3.5% fat-corrected milk yields in response to treatments. It is likely that the supplementation of NALM to mid to late lactating dairy cows may have shifted nutrient and energy utilization toward tissue gain rather than lactation, which resulted in a decrease in feed efficiency for lactation (P = 0.02). Overall results from the present study suggest that supplementing NALM to mid to late lactating cows can increase milk yield in a dose dependent manner with a shift of net energy partitioning toward milk production and body weight gain. In addition, supplementing NALM increased milk N output without affecting urinary N excretion.