One of the most important facilities required in the synthesis of an advanced mixed-mode system is the efficient and if possible automated analog design tool. In this paper an accurate method to determine the device sizes in an analog integrated circuit on the basis of genetic algorithm (GA) is presented. In order to evaluate the fitness of the circuit specifications in any iteration of the GA, HSPICE simulation is used. Examples in both time and frequency domains for an operational transconductance amplifier are presented. The simulation results confirm the efficiency of GA in determining the device sizes in an analog circuit.