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Air temperature in greenhouse is high for tomato plants in summer. It is important to maintain optimum temperature in greenhouse for yield. The cost of whole cooling in greenhouse is so expensive that local temperature control is suggested. Our previous research showed tomato plants with root cooling at 25 C increased yields compared with those with no cooling. The chilled water can be used for root cooling system in greenhouse. The objective of this research was to evaluate the root cooling system using cold water for growing tomato plants in summer. Tomato plants were grown hydroponically in greenhouse. The cooling water flows into PVC pipe which is installed under cultivation bed. Water temperature was maintained at 12 C. Inlet and outlet water in pipe and rockwool temperatures were measured. Our results showed that the rockwool temperature was maintained around 22C with high air temperature (30C) using root cooling system.