The aim of this study is to evaluate the effect of strengthen of soil under concrete slab on its thickness. Rigid pavement strength depends on the rigidity of concrete slab in addition to the effect of soil strength. This research tries to study strengthen of soil under rigid pavement using different compositions of coarse and fine aggregate. The term which expresses soil strength in the AASHTO design equation is the value of the modulus of subgrade reaction (K). Various compositions of fine and coarse aggregate have been tested in the lab. Each sample has been analysed in term of sieve analysis and California bearing ratio (CBR). Using CBR value for each sample and AASHTO equation, the modulus of subgrade reaction has been estimated. Using the estimated value of modulus of subgrade reaction and AASHTO design method for rigid pavement the slab thickness has been estimated for each case. Results have been analysed to get the effect of best composition of coarse and fine aggregate under rigid pavement.