Abstract: The liquid drop breakup problem was introduced to obtain and reveal the evolving property and mechanism of its two-phase interface. The early-time interface instabilities in supersonic aero-breakup of a liquid drop were investigated by numerical simulations. A combined analysis based on simulation results and linear-instability theory shows that both R-T (Rayleigh-Taylor) and K-H (Kelvin-Helmholtz) instabilities contribute the dominant disturbances originated from about the half way from the stagnation point to the equator. This is verified further with a specially modified simulation, which decreases the effect of K-H instability while keeps other flow properties unchanged.