ab initio method used on the mechanism of acetalization of 2-methoxybenzaldehyde. ab initio method is a quantum mechanical approximate calculation and derived directly only from theoretical principles. All geometry optimizations were performed using 3-21G and 6-31G* basis set with Hyperchem 8.0 software (windows version). The aim of this study was to focus on the study of the mechanism of acetalization of 2-methoxybenzaldehyde using hydrochloric acid as catalysts. The computational calculation not only provided possible reaction steps but also provided possible energy change in each step of the reaction mechanism of acetalization of 2-methoxybenzaldehyde. The result showed that 2-methoxybenzaldehyde (0 kJ/mol) has the lowest energy and electronegativity compared to acetal product (-17.43 kJ/mol) and a labile hemiacetal (448.33 kJ/mol) due to its stability and the influence of neighbour atom.