This study was conducted to develop a system and an control algorithm for control the environment of a plant factory. The greenhouse control system for environmental control was largely composed of a computer and a PLC. The screen of control program was composed of a greenhouse figure which was included machinery and equipments for greenhouse, the graph of environmental factors of inside greenhouse and the image of greenhouse. In order to reduce temperature change, the operation time of ventilation window was changed by 3 stage according to difference between a target and present temperature. When is heating, a temperature variation was shown to be $16.7{pm}0.8^{circ}C$. When is cooling, a temperature variation was shown to be $23.1{pm}0.6^{circ}C$. When is humidifing, a humidity variation was shown to be $39.3{pm}1.6^{circ}C$ %RH. An environmental control system and a control algorithm were proved that it was shown a good performance in a control accuracy. So a computer control system should be adapted to a control system of a greenhouse and a plant factory.