  We report a direct observation of temperature-induced topological phase
transition between trivial and topological insulator in HgTe quantum well. By
using a gated Hall bar device, we measure and represent Landau levels in fan
charts at different temperatures and we follow the temperature evolution of a
peculiar pair of "zero-mode" Landau levels, which split from the edge of
electron-like and hole-like subbands. Their crossing at critical magnetic field
$B_c$ is a characteristic of inverted band structure in the quantum well. By
measuring the temperature dependence of $B_c$, we directly extract the critical
temperature $T_c$, at which the bulk band-gap vanishes and the topological
phase transition occurs. Above this critical temperature, the opening of a
trivial gap is clearly observed.
