Selecting the most suitable motor sizes, gear boxes and structure under certain constraints or desired values such as payload, speed, deflections, total weight, etc. for a dynamic system is an exhaustive and time-consuming iterative process. To overcome this problem, a new- “evolving” conceptual design algorithm is developed. The suggested algorithm can be used for the conceptual design of any dynamic system including drive-train and structural optimization. To illustrate the suggested methodology, a robot manipulator, having 3 degrees of freedom, is selected as a case study. The objective function is minimizing the robot mass while satisfying the desired dynamic requirements and constraints of link deflections. A dynamic simulation environment for flexible body motion, containing 3 DOF robot manipulator drive-trains and flexible links, is developed in an evolving optimization loop. The lumped parameter estimation method is used to model the flexibility of uniform links in Simmechanics by allowing the estimation of deflections caused by the dynamic motion. Thus, both dynamic and structural simulations are made simultaneously in Simmechanics with no additional software. Hence, drive-trains and thickness of all links are simultaneously optimized by using the suggested evolving conceptual design algorithm.