Order picking is the most labor-intensive and costly operation in warehouse. Efficient storage assignment rules enormously affect the order picking efficiency. Warehouse managers are concerned about searching for the most effective way of managing order picking, minimizing the costs involved in terms of travel distance. The purpose of this research is to develop an efficient dedicated storage allocation method utilizing both the principle of group technology (GT) and the cube-per-order index (COI) storage policy in a maintenance environment warehouse. In this method, spare parts required for a particular maintenance work are grouped together on the basis of similarity of usages, even if they are dissimilar in size, shape etc. After that, the groups and parts within each group are assigned according to COI rule. The performance of the method is evaluated using simulation and compared with other sequence, frequency and group based dedicated storage methods. A genetic algorithm is embedded into the simulation program to determine the optimal traveling sequence that will minimize the travel distance. In the simulation study, it is found that the proposed method has the highest reduction in the distance travelled by the pickers.