In this paper we deal with a simultaneous optimal design problem of mechanical/dynamical parameters and a control system for robot manipulators. The mechanical parameters are evaluated from an index of manipulability based on Jacobian matrix of the robot manipulators. We propose an LMI based optimization method for the mechanism of the manipulator whose Jacobian matrix is an affine function on the length of each link. Dynamical parameters are evaluated by a closed-loop performance controlled by an LMI based gain-scheduled controller. We propose a new simultaneous optimal design method of mechanical/dynamical parameters and gain-scheduled controller using a numerical design algorithm that consists of an iterative LMI approximation of nonlinear matrix inequalities.