ABSTRACT The design of flexible pavement consists of determining the thickness of base and sub-base for the known strength of sub-grade soil. One of the main principles of providing the sub-base is to reduce the overall cost of the pavement with lesser strength material. The recommended sub-base materials, as per IRC 37-2001 standards are natural sand, moorum, gravel, laterite, kankar, brick metal, crushed stones, crushed slag and crushed concrete. Of these materials moorum is cheapest, easily procured and also, available at large quantity. As per IRC standards, the CBR value of any sub- base material should not be less than 20%. But in actual practice, the CBR valve of moorum as sub-base material often falls short of this mark. Soil improvement using fly ash alone as an additive is not satisfying the requirement mostly. To overcome these limitations, the tendency in the field is to choose gravel as an alternative material instead of moorum, which is not only costlier but also superior quality material whose strength is very much under utilized. Hence, this study examines and ensures the use of fly ash-moorum mix as a successful sub-base material by setting target CBR strength of 20% and achieving it by cement addition.