Petroleum refinery produces vacuum residue in a vacuum distillation unit as waste. Mixture of the vacuum residue and a conjugated double bond compound can be used as feedstock to produce activated carbon because the compound forms a stable mesophase at high temperature to allow polymerization of aromatic compounds in vacuum residue. Castor oil, which is available in tropical forest in Indonesia, can be dehydrated to form conjugated double-bond compounds. Polymerization can form a pitch with different extents of aromatization so that high surface pore area of the activated carbon can be achieved. The subsequent process was carbonization of the pitch to form activated carbon, where oligomerization and cracking of side chains occurred. The purpose of this study is to investigate effect of addition of dehydrated castor oil on aromatic growth during polymerization and voidage of activated carbon precursor and on carbon content and pore surface area of activated carbon. Co-pyrolysis to prepare activated carbon precursor improved aromatic content compared to the mixture without co-pyrolysis, but with short holding time at maximum temperature during co-pyrolysis, the addition of dehydrated castor oil of 10 and 15% gave small improvement. SEM-EDX analysis upon activated carbon indicates that only addition of dehydrated castor oil to vacuum residue of 15% exhibited very few voids in the activated carbon, which means that the aromatization growth filled nearly the whole voidage of activated carbon to form pores. BET analysis shows that increasing addition of dehydrated castor oil improved pore surface area from 733 to 1208 m2/g. SEM-EDX analysis of activated carbon shows that increasing addition of dehydrated castor oil increased carbon content from 74% to 85%.