Experiments on surface tension driven convection of high temperature potassium tantalate niobate (KTa1-xNbxO3, KTN) melts have been carried out in an in-situ observation system. There is a double loop axially symmetric vortex pattern. With the temperature increases, the convection can be transferred from one stable state to oscillatory state and then to another stable state. The oscillatory surface tension convection, which is related to the free surface deformation, can be characterized by the oscillatory main trunk and two unequal branches. The branches depress the large pressure gradient and compensate the return flow to stabilize the circular current. There is a linear decreasing relationship between the deformation angle theta and the oscillatory frequency f For KTN melts with higher Nb content, the convective pattern is not various, but a smaller degree of deformation can occur accompanied by a weaker oscillation process. The values of critical deformation coefficient S parameter for both the onset and the offset of oscillations decrease with the increase of the content of Nb. The critical values of oscillation satisfy with S-on . x approximate to 2.3 x 10(-3) and S-off . x approximate to 4.0 x 10(-3), respectively. (C) 2018 Elsevier Ltd. All rights reserved.