The objective of this study was to evaluate the effects of fish oil on lamb performance, carcass yield, ruminal fermentation, and leptin gene expression. Thirty-two lambs (24.10 kg initial BW, Katahdin × Pelibuey) were used in a completely randomized design. Dietary treatments were: 1) 0 fish oil and 30% corn (DM), 2) 1% fish oil and 25% corn, fish oil substituted for corn (DM), 3) 2% fish oil and 20.5% corn (DM), and 4) 3% fish oil and 17% corn (DM). The lambs were weighed on consecutive days at the beginning (d 0 and 1) and at the end (d 55 and 56) of the trial. Ruminal fluid samples were collected on d 56 to evaluate the ruminal fermentation pattern. The lambs were slaughtered on d 56; the perirenal adipose tissue samples were collected, and the carcass yields were recorded. The VFA, ammonia N and leptin mRNA expression were not affected (P > 0.05) by the dietary treatments. However, the DMI, ADG, final BW and the hot carcass yield (HCY) showed linear or quadratic responses (P < 0.05) as fish oil was increased in the ration; the estimated optimal responses were obtained with fish oil levels of 1.16% for the final BW, 1.03% for the HCY, 1.12% for the ADG, and 0.78% for the DMI. Additionally, the feed efficiency and the back fat showed a quadratic response (P < 0.05) as fish oil was increased in the diet; the estimated optimal responses were obtained with fish oil levels of 1.4% for feed efficiency and 1.6% for back fat. Increasing fish oil in the diet did not have a negative impact on ruminal fermentation or leptin mRNA expression; however, levels of 1% fish oil improved lamb performance and allowed for a reduction in the amount of grains in the rations.