Nanocrystalline particles of YB66 were prepared by plasma chemical process, using starting powders of YB4 and B, and the morphology was examined by high-resolution transmission electron microscopy (HRTEM). The average grain size of the YB66 particles is less than 100 nm. All the YB66 nano-particles are cubic in shape, suggesting that they have been formed through a direct coagulation from a vapor phase. Facetting at the cube corners occurs in most particles. The [0 0 1] and [0 1 1] lattice images of a cubic particle with edge-length of 84 nm were obtained. The simulated and experimental images match very well. A considerable amount of coarse, nearly spherical YB6 particles and unreached boron were additionally found by HRTEM.