The effects of ZSM-5 additive and riser reactor temperature on the product selectivity in fluid catalytic cracking of gas oil were systematically investigated in an adiabatic circulating riser reactor. In addition to the fact that ZSM-5 can substantially increase propylene and butene, it is found that ethylene yield linearly increases with the amount of ZSM-5 in the system. The ethylene is likely from the overcracking of C5 and C6 olefins. The effect of ZSM-5 on gasoline range olefins was clarified. In the temperature range of 794-839 K (970-1050 degrees F) studied in the current work, ZSM-5 additive always reduces C6 + olefins. The effect of ZSM-5 on C5= is different. C5 olefin increases at 794 K, but decreases at 839 K. Since C5 olefins account for about one-third to one-half of the gasoline olefins, the overall gasoline olefin contents were dominated by the behavior of C5 olefins. Compared with the effect of ZSM-5, the effects of temperature on light olefins are relatively small. Most of the olefin increases with increasing temperature were due to the increased conversion at higher temperature, rather than a selectivity shift. The yields of diolefins increase with temperature. The effect of ZSM-5 on gasoline diolefins was found to be minimal.