Abstract: The temperature oscillation and the free surface deformation are two essential phenomena for the fluid with free surface. In this paper, an annular pool with upper opening is constructed to obtain critical conditions of the onset of temperature oscillations and free surface oscillations when the horizontal radial temperature gradient is given. The experimental pool is cooled from the outer cylinder walls by 6 semiconductor coolers and linearly heated from the inner cylinder walls at the rate of 0.5℃/min, resulting in a horizontal radial temperature difference. The T-type thermocouple is used here to measure the temperature at a single point in the liquid layer and the free surface deformation is obtained by the CCD (charge-coupled device) displacement sensor at the same time. The temperature and the free surface begin to oscillate when the temperature exceeds a threshold value with the increase of the temperature difference. Experimental results show that for the same kind of silicone oil, the Ma_cr is proportional to the Bond number; the enhancement of the buoyancy convection can stabilize the whole flow; on the other hand, with the same thickness, the silicone oil of 1.5cSt has a larger Ma_cr than that of 2cSt; the decrease of the viscosity makes the flow more stable.