Thermocapillary flow instabilities in annular pool with different aspect ratios were investigated by the linear stability analysis based on the Legendre spectral element method. The energy analysis was applied to explore the underlying physical mechanism for the instability of thermocapillary flow. The results reveal two types of instability for different aspect ratios. The onset of instability is an oscillatory bifurcation in the cases of small aspect ratio whereas a stationary bifurcation in the cases of large aspect ratio. Moreover, the energy analysis indicates that the bifurcation of the flow is caused by the hydrodynamic (inertial) instability mechanism for low Prandtl number (Pr = 0.011), while by the coupled hydrodynamic effect and Marangoni effect for moderate Prandtl number (Pr = 1.4). (C) 2018 Elsevier Ltd. All rights reserved.