In this paper, we describe our computational approach to understand the factors influencing the selectivity of alkylation reactions over zeolite catalysts. Firstly, the shape, size and flexibility of the alkylated products are correlated to their diffusion behaviour inside zeolites. Secondly, the influence of pore architecture and the dimension of various large pore zeolites on diffusion characteristics of alkylaromatics are brought out. Further, we study the isomorphous substitution of Al, Ga or B in the place of Si and their consequent influence in the selectivity of alkylation reaction. We apply hard-soft acid-base (HSAB) principle which provides the properties to describe the acidity and reactivity of zeolites. The analysis of the results indicates that the shape selectivity in the alkylation reaction is also controlled by the molecular orbital properties in addition to the geometry constraints.