Abstract: The thermocapillary convection and its instability is an important problem in the microgravity fluid science. Its study will not only improve our understanding of the fluid behavior in microgravity conditions but also benefit the space and terrestrial applications such as the crystal growth and the film preparation. This paper studies the thermocapillary convection in thin liquid layers contained in an open rectangular cavity with differently heated sidewalls. In our experiments, a rectangular cavity of l = 52mm and w = 36mm is used, and the silicone oil with the kinetic viscosity of 1cSt is chosen as the working fluid whose Prandtl number is 16.2 at 25℃. The particle image velocimetry (PIV) is employed to observe and measure the flow structure in the thin liquid layers. Multiple flow states are observed within the parameter range examined. It is found that the transition routes depend on the thickness of the liquid layer. For thinner layers, as ΔT is increased, the flow structure in the vertical section transits first from the unicellular flow to the bicellular flow, and then to multicellular flow with several corotating rolls embedded in the main flow. And the number of the rolls decreases as ΔT is increased. The flow will eventually become time dependent and three dimensional if ΔT is even larger. While for thicker layers, the transition route is different. As ΔT is increased, the topology of the　flow structure in the vertical section changes little, but different flow states can be differentiated in terms of the flow structure in the horizontal section. When ΔT is small, the flow near y = 0 is two dimensional. As ΔT is increased, shuttle structures will occur, which are symmetric with respect to y = 0. But a larger ΔT will destroy the symmetry and turn the flow in to a 3D unsteady flow.