This project was to study the properties of basement soil which used portland cement laterite soil and slag as the main ingredients and to investigate its appropriate ration to be used for pavement improvement. By using 95% of laterite by weight and 5% of the mixed Portland cement and slag by weight the amount of portland cement to that of slag at 5% : 0% 3.5% : 1.5% 2.5% : 2.5% 1.5% : 3.5% and 0% : 5% by weight were mixed together. Then the derived mixture was put together with the laterite which was the major material. The study showed that the ration at 3.5% of portland cement and 1.5% slag to the laterite at 95% provided the best basement soil in quality. At this ration it saved the cement content used as well. At this ration it gave soil cement higher plasticity. Its CBR is high as well. Moreover it provided the standard unconfined compressive strength according to that defined by The Department of Highway of Thailand.