The influences of zeolite pore structure, dealumination and Na ion exchange on the product distribution were examined in order to explore the possibility of selective formation of multi-branched-chain aliphatics, such as iso-octane, in methanol conversion. On H-form mordenite, the selectivity of multi-branched-chain aliphatics was higher than those on other type zeolites such as ZSM-5 and Y zeolites. Modification of mordenite by using both Na ion exchange and dealumination promoted the formation of multi-branched-chain aliphatics. The effect of the modifications in the formation of multi-branched-chain aliphatics is discussed in reference to methylation of 1-hexene with methanol.