The displacement effect in the theory of body motion in waves is discussed from the view-point of perturbation theory. First of all, in order to clarify the physical meaning of the displacement effect, a body motion in an unbounded fluid is discussed. Then the first order problem of a body motion in a uniform flow with free surface and the second order problem of a body motion in a still water with free surface are discussed. It becomes clear that the displacement effect appears in the velocity potential by making use of the space coordinate, and then it cancels out the displacement effect which appears in the pressure equation, when the pressure on the body is calculated.