If a particle moves along a circular track under the action of central force F=-c/rⁿ,the track is stable when n<3. If the potential energy of a particle moving along a circular track is U(r)=-km/r^p,the track is stable when 0<p<2.This paper points out the difference in the instability of the track under the circumstances of n=3 and n>3,and discusses the relationship between two conditions of stability respectively given by force and potential energy.