This study was conducted to develop the on-line monitoring system for plant growth responses. The system consisted of two parts. One system was the measuring system and the other was its controlling system. The established measuring systems were the ultrasonic wave sensor driver for height of plant, the potentiometer for diameter of plant stem, and the weighing system with strain gage application for plant weight. Also, computer program for measurement and controlling was developed, and the whole system was tested by the fabricated plant, and the actual plant growth responses were monitored by the system. When monitoring the actual plant growth responses, even the small amount of plant growth resposes could be measured by the system within tolerable error ranges.