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The instability of a crystal–melt interface during the directional growth of pure antimony was studied using an in-situ observation technique. The morphology of the crystal–melt interface was planar at a low growth velocity, while the interface transitioned to wavy and then zig-zag faceted at 60 μm s−1. Calculating the thermal field at the crystal–melt interface using a diffusion equation showed that the temperature gradient in the melt becomes negative when the moving velocity of the interface is higher than 52 μm s−1. These results demonstrate that perturbations introduced into the planar interface are amplified by a negative temperature gradient and these amplified perturbations result in zig-zag facets.