Flame spread phenomena over an oil slick floating on water was studied from a view of the surface tension flow. Two series of experiments were conducted. The first one was done for clarifying the critical Marangoni number for causing the surface tension flow. An oil slick floating on water was heated by a nichrome wire electrically in the experiment. The other experiment was done for ascertain whether the critical Marangoni number practically affects the flame spread rate over the oil slick. It was found that the critical Marangoni number for an oil slick floating on water was fairly large, e.g. 7600 ± 730 for decane. However, the Marangoni number was larger than the critical value at sub-flash temperatures for a spreading flame over a thin liquid layer practically. Therefore, the surface tension flow existed in front of the spreading flame, and it caused the flame pulsation.