Two-stage pyrolysis of Taching vacuum residue and Arabian Light atmospheric residue was carried out using a flow-type reactor to manufacture lower olefins. In this method, heavy oils were pyrolyzed at 440/degree/C to produce cracked oils, which were carried to a high-temperature zone (700-800/degree/C) to undergo subsequent pyrolysis. The operating conditions at the high-temperature zone, temperature range of 700-825/degree/C and residence time of 0.35-1.5 s, were examined on the effect of the respective yield of olefins and aromatics. The highest ethylene yield of 27 wt% against feed was obtained at 800/degree/C, 0.4-0.7 s for the Taching sample and 23 wt% at 800/degree/ C, 1.0 s for the Arabian Light sample. The highest propylene yield was about 12 wt% for both samples. Total yield of C/sub 1/ to C/sub 4/ hydrocarbon gases showed a maximum, about 60%, both at 800/degree/C, 0.35 s and 750/degree/C, 1.0 s for the Taching residue. 7 refs.