In this article, we report the theoretical and experimental investigations on a dynamic mode force microscopy for detecting the force gradients in two directions. The motion of the probe oscillated in two directions was analyzed on the basis of the mechanical dynamics theory to provide the ability of detecting the force gradient in vertical and lateral directions separately. When the spring constants of the cantilever probe in two orthogonal directions were different, the gradients of the attractive forces acting in two orthogonal directions were detected separately from the resonant frequency shifts. The theoretical consideration was confirmed by the experiment in which the gradients of electrostatic force acting on a fine electrode sample were measured.