Water pollution of coast has a significant impact on the fish farm and fisheries. For solving the water pollution problems the fish farms are moving to the open sea. The fish farms in open sea have to operate by the automatic feeding system and remote monitoring system for safety and management. This paper describes an automatic feeding system for fish farms in open sea. Water temperature and fish weight will change depending on the amount of feed. And the fish farm temperature is changed extremely in open sea than on land side. This paper described that the feed amount is calculated automatically according to temperature, fish weight, and the automatic feed system. And the performance of automatic feed system is verified with test model for operation test.