Determining the motion using optimal technique about traveling time and trajectory planning has been studied often in recent years, but the study of determining the optimal robot dimensions is rare, the authors attempt to find out the least driving torques and energy as the optimization of link length ratio referred to 2R SCARA and 3R robot manipulators. For the given linear path with triangular velocity profile, the inverse kinematic and dynamic problems are examined in order to lead into solution of problem, which is suggested for optimal design of link lengths. Accordingly, optimal link length ratio is obtained with respect to each case.