In this paper, we propose a method to efficiently select module combinations of 6 DOF modular manipulator. In order for a robot to follow a given work trajectory, the solution of the inverse kinematics must be obtained through numerous iterative calculations using numerical solutions. It takes a lot of time to apply these iterative calculations to various module combinations. Excluding module combinations that are unlikely to satisfy the given task before enter-ing a numerical iteration process can save lot of time. For this purpose, two types of workspaces a reachable workspace (considering only the position of the end of the modular manipulator) and a dexterous workspace (considering the position and direction) are used filter out some incompetent module combinations using the mathematical criteria per each workspace. Finally, this algorithm was applied to the program that selects the optimal module combination to verify the time efficiency.