The applicability of the P.I.D. control algorithm for an identified soil temperature system1) was investigated using both experiments and computer simulations.Results:1) The P.I.D. soil temperature control experiments showed that although every four optimally adjusted P.I.D. parameter sets lead the controlled variable, (the soil temperature at the logarithmical middle point) to the stepwise desired value in the steady state, the transient response curves are differ greatly when four sets of optimal P.I.D. parameters from the C.H.R. method are used. The simulated P.I.D. soil temperature controls were similar to the above experiments except for slight differences in detail. This implies that investigations of P.I.D. soil temperatue control systems can be carried out using computer simulations composed of the mathematical model for a soil temperature system developed in a previous paper1) .2) Not only was it determined that P.I.D. soil temperature control can be performed using a digital computer, but that the P.I.D. soil temperature control system, itself, and the soil temperature distributions can be investigated with computer simulations while the soil temperature is P.I.D. controlled, if the digital computer possesses the state space simulation model of the P.I.D. soil temperature control system developed in the present paper.