Root growth of rubber trees (Hevea brasiliensis Muell. Arg.) was assessed by minirhizotron technique in 9 year-old rubber trees (RRIM 600 clone). spacing 7 x 3 meters. The following data, growth and change the root, soil moisture, leaf area index, were recorded at 2- week interval for 17 months. The analysis of growth model a Logistic growth curve It was found that tube length 100 and 150 cm. have root growth rapidly 2-3 months, respectively. The changes of root in years, it was found that the length of pipe with 100 cm. developed a deeper root growth at 0-10 cm at soil profile. For tube length 150 cm, the root system of rubber tree tended to proliferate in shallow soil layer (0-70 cm). In May, there were the profiles of root growth were higher than that of 10 cm. And it was found that soil moisture are the factors that affect the growth of root. By soil moisture increased root growth will increase. While the moisture reduced root is death and root volume reduced. It also found that the root contains the highest growth after a new crack