Thermal isomerization of (all‐E)‐lycopene to the corresponding Z‐isomers was investigated in edible vegetable oils; perilla, linseed, grape seed, soybean, corn, sesame, rapeseed, rice bran, safflower seed, olive, and sunflower seed oil. Purified (all‐E)‐lycopene from tomato oleoresin was converted to Z‐isomers in the range of 44.8–58.8% content, and the remaining ratio of total amount of lycopene isomers without decomposition were ranged from 38.8 to 79.6% after heating at 100°C for 1 h in the vegetable oils. Both values were exceedingly high in sesame oil: 58.8% of total Z‐isomers content and 78.3% of remaining lycopene. In particular, (5Z)‐lycopene, which has higher bioavailability and antioxidant capacity as well as greater storage stability among the Z‐isomers, was notably increased in that oil; approximately threefold higher than the average of the other vegetable oils. Practical applications: Lycopene offers many health benefits such as decreased risk of cancer and arteriosclerosis. The Z‐isomers of lycopene, occurring in processed tomato products, are more bioavailable than the all‐E‐isomer which is a major configuration form in raw tomatoes. This study has developed an efficient production method for Z‐isomers of lycopene by heating (all‐E)‐lycopene in edible vegetable oils, in which neither organic solvents nor food additives are used. It would be practically feasible to utilize this procedure for the food, drink, and dietary supplement manufacturing, as well as for daily cooking at home. Thermal isomerization of (all‐E)‐lycopene to Z‐isomers was investigated in edible vegetable oils. In sesame oil, (5Z)‐lycopene, which has higher bioavailability, antioxidant capacity, and greater storage stability among the Z‐isomers, was increased significantly.