•Facet formation during solidification of pure Sb was directly observed.•The faceted solid–liquid interface of pure Sb is bounded by {11¯02}•Mullins-Sekerka instability was not identified on the Sb{11¯02}•The α factor in the Jackson model does not explicitly predict the growth form. In contrast to metals and semiconductors, few studies have focused on the faceting behavior of semimetals, and the atomic configuration of the solid–liquid interface is still incompletely understood. In this study, to obtain insight into the growth dynamics of semimetals, we directly observed solid–liquid interfaces during directional solidification of pure antimony. Morphological transformation of the solid–liquid interface depends on the direction of solidification: a zig-zag shaped solid–liquid interface is formed after the onset of instability in the direction of solidification normal to {312¯925}, while a curved interface becomes planar and no zig-zag facet formation is identified in the direction of solidification normal to {811¯324}. By combining in-situ observation and ex-situ electron-backscattered diffraction techniques, we showed that zig-zag shaped faces formed after the onset of instability at the solid–liquid interface are bounded by equivalent {11¯02} planes. The fact that the slowest growth kinetics occurs on the {11¯02} plane was also confirmed by measuring the growth velocity of the solid–liquid interface and the rate of temperature decrease in an observation furnace. Calculations based on thermal diffusion equations show that the Mullins-Sekerka instability does not occur on the {11¯02} plane, indicating that the growing {11¯02} plane at the solid–liquid interface is faceted.