A process for enhancing the yield of C3 olefin in fluid catalytic cracking of hydrocarbon feedstock , the process comprising: a) contacting a hydrocarbon feedstock with a fluidizable solid micro-spherical cracking catalyst in a primary reaction zone of a riser to produce cracked hydrocarbon products and spent catalyst; b) separating the spent catalyst from the cracked hydrocarbon products and stripping the spent catalyst with steam to remove the hydrocarbons entrapped inside the pores; c) separating a C4 hydrocarbon fraction of the cracked hydrocarbon products to obtain a recycle C4 hydrocarbon fraction; d) burning off the coke deposited on the spent catalyst in a catalyst regenerator to obtain a hot regenerated catalyst; e) recycling a part of the hot regenerated catalyst into the primary reaction zone and the remaining part is cooled in a catalyst cooler to obtain a cooled regenerated catalyst, which is recycled to a secondary reaction zone located at upstream of the primary reaction zone of the riser, wherein flow of the hot regenerated catalyst into the primary reaction zone is controlled to achieve the temperature at the exit of riser in the range of 500 to 650 °C; f) contacting the recycle C4 hydrocarbon fraction with the cooled regenerated catalyst in the secondary reaction zone of the riser at a weight hourly space velocity in the range of 5 hr -1 to 10 hr -1 for oligomerization reaction of the olefinic molecules of the recycle C4 hydrocarbon fraction to obtain oligomers, wherein the hot regenerated catalyst to the secondary reaction zone is cooled in the catalyst cooler and flow is controlled to achieve the temperature in the range of 350°C to 450°C inside the secondary reaction zone; and g) lifting the oligomers into the primary reaction zone of the riser in order to get cracked to produce C3 olefin, wherein a part of the spent catalyst from a reactor vessel is directed to the catalyst cooler to maintain carbon content of the catalyst in the range of 0.3 wt% to 1 wt % in the secondary reaction zone. a) contacting a hydrocarbon feedstock with a fluidizable solid micro-spherical cracking catalyst in a primary reaction zone of a riser to produce cracked hydrocarbon products and spent catalyst; b) separating the spent catalyst from the cracked hydrocarbon products and stripping the spent catalyst with steam to remove the hydrocarbons entrapped inside the pores; c) separating a C4 hydrocarbon fraction of the cracked hydrocarbon products to obtain a recycle C4 hydrocarbon fraction; d) burning off the coke deposited on the spent catalyst in a catalyst regenerator to obtain a hot regenerated catalyst; e) recycling a part of the hot regenerated catalyst into the primary reaction zone and the remaining part is cooled in a catalyst cooler to obtain a cooled regenerated catalyst, which is recycled to a secondary reaction zone located at upstream of the primary reaction zone of the riser, wherein flow of the hot regenerated catalyst into the primary reaction zone is controlled to achieve the temperature at the exit of riser in the range of 500 to 650 °C; f) contacting the recycle C4 hydrocarbon fraction with the cooled regenerated catalyst in the secondary reaction zone of the riser at a weight hourly space velocity in the range of 5 hr -1 to 10 hr -1 for oligomerization reaction of the olefinic molecules of the recycle C4 hydrocarbon fraction to obtain oligomers, wherein the hot regenerated catalyst to the secondary reaction zone is cooled in the catalyst cooler and flow is controlled to achieve the temperature in the range of 350°C to 450°C inside the secondary reaction zone; and g) lifting the oligomers into the primary reaction zone of the riser in order to get cracked to produce C3 olefin, wherein a part of the spent catalyst from a reactor vessel is directed to the catalyst cooler to maintain carbon content of the catalyst in the range of 0.3 wt% to 1 wt % in the secondary reaction zone. <EOS>