The role of para rubber plantation on streamflow regulation in southern Thailand was studied by using 15 watersheds. The result found that the maximum peak flow of small watershed was the highest, but the lowest peak was less than middle and large one. Also the large and middle watersheds contributed more regularity of water flow than small size. Based on percentage of para rubber cover, the investigation has been divided into 4 groups

they are less 25 %, 25-33 %, 33-50 % and more than 50 %. It was found that the increasing of para rubber cover caused the decreases of peak flow, the percentage of baseflow to rian-water, and baseflow recession cofficient, while the direct runoff and its coefficients had uncertain direction. Furthermore, the potential of streamflow as related to para rubber cover showed unreliability but found runoff efficiency of natural forest nearly the same as protected forest. The study has also conducted to analyze the relationships between streamflow and direct-flow recession coefficients to percentage of para rubber cover, rainfall, relief ratio and size of watershed. It indicated the positive relation between streamflow coefficient to relief ratio and to watershed area, while direct-flow coefficient found positive relation to para rubber area, rainfall, relief ratio and size of watershed. However, all variables had very less influence.