To examine the effect of dietary ${omega}3$ fatty acids on cardiac mitochondrial inner membrane lipid composition and respiration, rats were fed diet containing 10% (w/w) mackerel oil and perilla oil for 24 days. The other two groups of rats were fed soybean oil and mackerel oil plus soybean oil (3:1) mix to compare the effect. Mitochondrial inner membrane phospholipid of mackerel oil group had higher content of phosphatidylcholine than phosphatidylethanolamine, the phenomena of which was reverse in vegetable oil groups. The amount of cardiolipin was the highest in mackerel oil group. Fatty acid composition of phospholipid was characteristic in each group, i.e., monounsaturated fatty acids and $C_{20-22}$ (${omega}3$) fatty acids were higher in mackerel oil group, while $C_{18:2}$ (${omega}6$) and $C_{20:4}$ (${omega}6$) fatty acids were higher in soybean oil group, but neither $C_{18:3}$ (${omega}3$) nor $C_{20-22}$ (${omega}3$) fatty acids were significantly high in rats fed perilla oil, in which $C_{18:3}$ (${omega}3$) comprises 60% of total fatty acids. This result suggests that elongation of $C_{18:3}$ (${omega}3$) to $C_{20-22}$ (${omega}3$) was not very active. These changes in the structure of mitochondrial inner membrane were not accompanied by any notable alteration in respiratory function of heart mitochondria.