An ex-vessel–type control element drive mechanism (CEDM) is installed on the nozzles of a reactor vessel closure head. However, there has been a demand for locating CEDMs inside the reactor vessel because doing so provides many benefits such as eliminating rod ejection accidents, minimizing pressure boundary penetration, and simplifying the reactor head area structure. Accordingly, an in-vessel CEDM has been developed to operate inside the nuclear reactor. Developing an in-vessel CEDM is very challenging because of the harsh environment of high temperature, high pressure, high radiation, and submerged condition. During intense research work for several years, KEPCO E&C designed an in-vessel CEDM and studied its feasibility by electromagnetic analysis and thermal analysis. Then, a prototype of the in-vessel CEDM was manufactured to prove its performance and operability at normal reactor operating conditions. This paper introduces key design work and validation tests of the in-vessel CEDM including an in-vessel position indicator.