To increase the light olefins selectivity of catalytic pyrolysis catalyst for heavy oil processing, the effects of large pore zeolite additions on the selectivity to light olefins (ethylene and propylene) were studied in a micro-activity test (MAT) unit at 625 degrees C by using Daqing heavy oil and n-decene/n-decane as feedstocks. Rare earth containing ultra-stable Y, HP and four types of alkali-treated HP with different pore size distributions were employed as the large pore zeolite components. The yields Of C-2-C-3 light olefins showed a volcano trend with the increasing amount of large pore zeolite additions. They reached up to 24.5 and 26.7 wt%, respectively, when an optimum combination of zeolites ZSM-5 and RE-USY or ZSM-5 and alkali-treated HP was used. Moreover, increasing the pore size of large pore zeolites also led to the increases in the yields of light olefins, the maximum total yields of ethylene and propylene reached up to 26.7 wt% when the total pore volume of the zeolite H beta added was 0.452 cm(3) g(-1). (C) 2007 Elsevier B.V. All rights reserved.