Small side thrusters having the rotor diameter less than 200 mm for ships are sometimes operated near the draft and the rotational speed of the rotor is high to keep its thrust. Therefore, it is said that cavitation may occur in the small side thruster. It is important to show whether cavitation will occur and the influence of the cavitation on the thrust should be clarified. In this research, visualization of cavitation in the small side thruster is conducted by the high speed camera and numerical analysis in high cavitation number region is compared to the visualization results to validate the numerical analysis results. After that, the thrust and cavitation behavior in wide cavitation number region including small cavitation number are investigated by the numerical analysis and its results are shown in this paper.