In this paper, the transition mechanism from steady to oscillatory flow of Marangoni convection is investigated. Two-dimensional simulations for high Pr number fluids with several liquid bridge sizes were conducted in order to clarify the effect of temperature distribution on free surface upon the transition phenomena. The dependency of liquid bridge size on the onset of oscillatory flow was also evaluated. The results show that the formation of velocity distribution on the free surface is related to liquid bridge size and that it has a great influence on the onset of oscillation. From these results, a basic model for understanding the transition mechanism of Marangoni convection is proposed. In this transition model, the temperature distribution on the free surface was evaluated over a wide range of Pr numbers. A useful dimensionless parameter which indicates the onset of oscillation, the effective Marangoni number, is also proposed and evaluated.