We investigated experimentally convective flow phenomena of Bi12SiO20 melts with Prandtl number Pr = 26. Schilren technique coupling differential interferential microscope was employed to in-situ observation of the convective flow. The test section was a loop-shaped Pt wire heater. Optical evaluations in connection with thermocouple (Pt90wt%-Rh10wt%) measurements made it possible to get the new transition ideas of flow structure in the considered convective system. In this article we deal with the phenomenon ranging from steady spatial phenomena to oscillatory behavior. Convective flow occurs in the form of an axially symmetric pattern (ASP). In the steady convection stage, the flow configuration appears as a fish-tail shape which comprises of a main trunk and two branches. The width of the main trunk decreases with rise in temperature, and in the main trunk the amplitudes of the hydrothermal waves increases. The critical Marangoni number Ma(c) is 22433 for the transition to oscillatory convection. The oscillatory convection occurs in the form of a non-axisymmetric pattern, that separates the flow configuration into two sectors. At one end of ASP the hydrothermal wave originates and at the other end of the ASP it vanishes. The oscillatory frequency was obtained at 5-7 Hz. (C) 2004 Elsevier B.V. All rights reserved.