A process and apparatus for catalytic cracking of hydrocarbon feedstock employing circulating fluidized bed reactor-regenerator configuration for maximizing the yield of propylene (C3 olefin) is disclosed. The apparatus comprises two reaction zones operating under different temperature and weight hourly space velocity (WHSV), one primary zone for cracking of hydrocarbon feedstock and other as secondary zone for cracking of C4 fraction produced from the cracking of hydrocarbon feedstock in the primary reaction zone, optionally admixed with C4 stream from external source. Two dedicated conduits equipped with valves for control of catalyst flow rate are provided to supply the hot catalyst from a common catalyst regeneration zone wherein the catalyst flowing though conduit connected to the secondary reaction zone is cooled employing a heat exchanging device. The lower temperature achieved in secondary reaction zone on account of exchange of heat along with lower weight hourly space velocity (WHSV) selectively promotes oligomerization of C4 fraction before being cracked to produce C3 olefin in the subsequent portion of the reaction zone (primary).