A process for conversion of hydrocarbon feedstock into lighter olefins of C2 to C4 carbons, the process comprising of cracking the hydrocarbon feedstock in a reactor in the presence of a catalyst. The catalyst for short contact time catalytic cracking process of heavy hydrocarbons having contact time less than 1 second to produce light olefins of C2 to C4 carbon in the range of 40 to 60 wt % on fresh feed basis in a fluidized bed reactor which is concentric downflow reactor in presence of catalyst consisting of ultra-stable Y zeolite in the range of 5-10 wt %, 4 to 8 wt % of pentasil zeolite, 2.5-5 wt % of bottom selective material, 0.5-2 wt % of rare earth and 75-88 wt % of support material.