Root dry matter distribution of tomato in rockwool slab of hydroponics was investigated on the Japanese ‘SHIFUKU’ and Dutch ‘GRACE’ variety. However root distributed from the base of the stem almost concentrically, more amount of roots were concentrated at the space between the plants than the other space. Protocol for estimation of root dry matter in rockwool was proposed. Only one week was necessary for estimation of the root dry matter of 20 tomato plants including 3 days drying process.