We propose a novel method to control a lot of microorganisms by using visual feedback for micro-robotic application. Our goal is to control a cluster of microorganisms as microscale smart robots for various applications, since microorganisms have efficient actuators and accurate sensors. Compared with single-cell level actuation methods proposed before, this method has the following advantages: (1) A cluster of microorganisms can cancel stochastic perturbations of behavior by taking the average; (2) A cluster of microorganisms can be more powerful than a single microorganism, and thus it can perform a variety of tasks. A visual feedback system is constructed to control cells using galvanotaxis (response to the electric field). Experimental results show the feasibility of visual feedback control of a paramecium cell cluster. In addition, Micro-manipulation of small objects by the cell cluster is also demonstrated.