Microdiffraction is used to characterize grain boundaries and misorientation between small particles. The incident beam and the trace direction of a vertical contacting boundary can be determined from the diffraction spots and the Kikuchi lines formed simultaneously by a small convergent probe of the electron beam. The bicrystal rotation can be obtained and thus the boundary structure of contacting particles can be estimated. The results for small particles of cobalt oxide showed that the accuracy of the method, which is mainly dominated by the determination of the trace and the verticality of the boundary, is comparable to that of the standard stereographic method, but the measurement and interpretation procedures are greatly simplified. This method can be applied for various crystalline materials including metals and alloys, ceramics, and semiconductors provided that the particle size is around 100 nm.