Tritiated water from fusion power reactors will be the next major issue when fusion technology comes fully onstream. Effective radiation protection measures will be implemented when the scope of its behavior is well understood. To understand tritium behavior in the environment, komatsuna was cultivated in tritium-contaminated peat soil. It was indicated experimentally from water immersion experiments that the amount of tissue free water tritium in komatsuna depends on the tritium concentration in the soil and that the concentration in stems and leaves in komatsuna decreases as the tritium concentration in the soil decreases. The amounts of tritium retained in the roots were much less than that in the stems and leaves.