The purpose of the present investigation was to study the water relation and drought resistance of lowland and upland rice in various soil moisture conditions. The plants of two representative varieties “Lao Lai Ching” (lowland rice) and “Nang Ton Xao” (upland rice) were grown in pots with three different conditions; namely: 1. Flooded state with a water layer of 5 cm depth. 2. Moistened state with soil moisture amounting to 70%–75% of the full water-holding capacity. 3. Moistened state with soil moisture amounting to 45%–50% of the full water-holding capacity. The results obtained lead to the following conclusions: It seems that no intrinsic difference in the water relation exists between lowland and upland rice. Both varieties carry on their normal life activities in flooded condition. With regard to the adaptability to upland condition there are distinct differences between them. Cultivation in the moistened state without flooding leads to a poor yield of grain of both varieties, but the reduction in yield being greater in “Lao Lai Ching”. The yield of upland rice is not so much influenced by soil moisture as that of lowland rice. At droughty condition the growth of the upland rice plants far surpasses that of lowland ones, especially in the case of root-system. In upland condition the total water content of leaves is found to decrease, but the degree of decrease is rather greater in “Lao Lai Ching” than in “Nang Ton Xao”. Thus the water dificit of the former is somewhat larger than that of the later. The change of free water content shows the same tendency as the total water content, except that the difference between two varieties is still more significant. The free/bond water ratio of upland rice is also higher than that of lowland rice. In the case of decrease of soil moisture exomosis of electrolytes from leaves increased. However, upland rice is not so sensible to decrease of soil moisture in moistened state. The increase of exomosis owing to the scanty water supply is not so much in “Nang Ton Xao” as in “Lao Lai Ching”. This difference is more marked when exomosis was carried on at high temperature (50 ℃), which indicates that the heat resistance of upland rice is somewhat higher than that of lowland rice. The data presented here supports the suggestion of Prof. Ting, who believes that lowland and upland rice are two ecotypes produced by the edaphic conditions of lowland or upland in the region of cultivation. The relationship of the plant growth, drought resistance and the physiological indicators of water supply has been discussed.