Previously we showed that supplementation of a corn silage-based diet with up to 3% of linseed oil (LO) reduced enteric CH₄ production without adverse effects on DMI and milk production. However, a higher supplementation level (4%) impaired DMI and milk yield. In the same study, we also examined the effects of supplementing increasing amounts LO on N balance of dairy cows fed corn silage-based diet. For this purpose, 12 lactating, multiparous Holstein cows (DIM = 84 ± 28; milk yield = 42 ± 4.6 kg/d) were used in a replicated 4 × 4 Latin square design (35-d period, 14-d adaptation). Cows were fed for ad libitum (5% orts, on an as-fed basis) a corn silage-based TMR (61:39; forage:concentrate ratio) not supplemented (control) or supplemented with 2, 3 or 4% LO (on DM basis). Intake of N, N excretion (fecal and urinary) and milk N secretion were determined over 6 consecutive days, while ruminal NH₃ concentration was determined over 2 consecutive days. Data were analyzed using the MIXED procedure (SAS) and differences between treatments and the control were declared significant (P ≤ 0.05) using Dunnett’s comparison test. Ruminal NH₃ concentration was unaffected by adding LO at 2 and 3% to the diet, but decreased at 4% LO. A decline in N intake was observed when LO was included at 2, 3 and 4% in the diet (−6, −8 and −19%, respectively). Nitrogen excretion (g/d) in urine and feces also decreased with increasing inclusion of LO in the diet. However, when expressed as a proportion of N intake, excretion of N in feces and urine was not affected by adding LO to the diet. Milk N efficiency (g milk N/g N intake) was higher for cows fed LO-supplemented diets (averaged 31%) compared with cows fed the control diet (26%). Results show that diet supplementation with up to 4% of LO reduced N intake, had no effect on fecal and urinary N excretion (as a proportion of N intake) and improved milk N efficiency of dairy cows. It is concluded that supplementing a corn-silage based diet with up to 3% of LO reduced the amount of N excreted in the manure and increased milk N efficiency without negative effects on DMI or milk yield.