The reported structural phase transition in YMnO3 from P63cm to P63/mmc space group occurs at 1258 ±14 K. For low temperature phase (space group P63cm) the phonon modes are found to be stable (Fig. 2) in the entire Brillouin zone. However, we find unstable phonon mode in the high temperature (space group P63/mmc) phases at M (0.5 0 0) and K (1/3 1/3 0) points. These unstable modes are highly anharmonic in nature and become stable at higher temperature due to anharmonicity. The transition from ferroeletric (space group P63cm) to paraelectric phase ((space group P63/mmc)) is expected to be driven by phonon anomalies. The low energy phonon modes up to 20 meV are expected to be most affected by the phase transitions. We propose to measure temperature dependence of low energy zone centre and zone boundary phonon modes from 300 K and 1300 K.