Abstract. In this paper we generalize minimal p-divisible groups defined by Oort to minimal F -crystals over algebraically closed fields of positive characteristic. We prove a structural theorem of minimal F -crystals and give an explicit formula of the Frobenius endomorphism of the basic minimal F -crystals that are the building blocks of the general minimal F -crystals. We then use minimal F -crystals to generalize minimal heights of p-divisible groups and give an upper bound of the isomorphism numbers of F -crystals, whose isogeny type are determined by simple F -isocrystals, in terms of their ranks, Hodge slopes and Newton slopes.