The energy aberration of crystal monochromator in the case of continuous X-ray spectrum, for instance EXAFS, has been discussed. Based on the fact that the intensity of continuous X-ray varies slowly with the energy and can be treated as a constant, we propose a method of calculating average resolution for flat crystal and the Johann and Johansson arrangement. We have also pointed out some inappropriate points in analyses on the reflect geometry of Johann arrangement described in previous publications. It is shown that Johann crystal can provide a better resolution than that commonly believed.