Abstract:  Plant growth could be directly observed by realtime precision measurement of tree diameter. So the realtime precision measuring device for tree diameter was developed in this study. The mechanical bracket and circuit system of each part had been designed firstly， and the realtime precision measuring device was then completed. Then， the diameter of Araucaria heterophyll had been measured by this device during sunny day， rainy day and cloudy day， respectively. And the week change law of tree diameter was summarized. The results showed that this realtime precision measuring device could be well used for measuring tree diameter.

Key words: tree diameter, tree diameter growth, realtime precision measuring