A method of operating a nuclear reactor adapted to use cross-shaped control rods. The control rods are grouped into a plurality of groups having a first group consisting of a control rod located at the center of the core, a second group consisting of 8 control rods surrounding the control rod of the first group, a third group consisting of control rods located adjacent to and outside of the control rods of the second group, and other groups determined successively in the same manner as in the third group. During the operation period other than the period in which the number of the control rods to be inserted for realizing the critical condition of said nuclear reactor is less than 6, the nuclear reactor is operated with control rod patterns in which the inserted control rods are selected from control rods of alternate or every other groups of said groups so as to include at least a pair of control rods which are located in a similar manner to that of the KNIGHT's movement on a chess board.