Microclimatic studies of the habitate indicate that the thermal conditions of the plant environment vary not only from time to time but in various portions of the plant. And the osmotic pressure of sap depends on the temperature as well as the concentration. The author insists that there are the influence of the temperature change on the osmotic system and the effect of the temperature gradient in relation to the translocation of substances in plants. This report gives an account of its theoretical base grounded by the theory of osmotic pressure. 1) The equilibrium of water absorption between the solution filled in the tube and the cell sap of the surrounding cells was discussed, assuming the cell membranes are semipermeable. Theoretically the suction pressure of the cells varies with temperatures. The turgor which is affected by the water absorption of the cells, therefore, also changes with temperatures. 2) Next, the above relations are applied to the osmotic system in a sap tube sorrounded by cells whose membranes are semipermeable and where a temperature gradient exisists in the tube, assuming the solution is uniform in concentration. Then the solution where the temperature is higher becomes more concentrated than the lower. If we consider that when the solutes are transferred by diffusion, the molecules of the solvents move reversely at the same time, there must be a tendency that the solutes move from the higher temperature portion to the lower in the tube along which a temperature gradient exisists and the solvents move reversely. The author will name it the effect of temperature system on the translocation of substances in plants. The sieve tube of plants may be compared as the sap tube surrounded by the cells whose membranes are semipermeable. The effect of thermosystem, therefore, must have a significance to the translocation of assimilates and other substances in the sieve tube especially.