To achieve high efficiency and cost reduction, large heat input welding such as submerged arc welding (SAW) and metal inert gas arc welding (MIG) using large currents are employed. However, the large heat input given to the weld may produce coarse grains in the HAZ and a reduction of toughness. To avoid these problems, narrow gap welding (NGW) which is effective in reducing heat input and achieving high productivity is often employed. However, the pear-shaped bead cracking may form according to the welding condition in the case of narrow gap welding. Therefore, the welding conditions must be carefully selected to prevent the cracks. In this study, the influences of the heat input parameters, such as heat input width, heat input depth and the total amount of heat input on the pear-shaped bead crack formation under narrow gap welding are investigated using the Thermal-elastic-plastic FEM with temperature dependent interface element. Furthermore, to find the optimum values of the heat input parameters, the optimization system which can consider the formation of pear-shaped bead cracking is developed. As optimization methods, Complex Method (CM) and Genetic Algorithm (GA) are adopted. Using these methods, the ideal heat input distribution to achieve deeper penetration with small heat input and without cracking can be computed. As a result, the superiority of the welding using narrow gap can be shown in the viewpoint of heat input efficiency.