This paper is dealing with a smart actuator for a modular robot. Modular robot means that robot has modular actuator in each axis. Smart actuator consists of harmonic reducer, servo motor, electric brake, multi-turns absolute encoder, drive, network board. All of them are hollow shaft type module. The robot which is linked to the smart actuator with EtherCAT and SPI communication is easily able to assemble and disassemble or reconfigure it. We try to implement a smart actuator and this tries to be verified through an experiment in order to implement the robot.