An experimental study has been made of the enhancement of heat transfer for water at a subcooling of 70, 40, 20, and 10 K and for ethanol at a subcooling of 60 and 30 K in a narrow vertical rectangular channel (20 mm wide, 2 mm space and 200 mm long) through which air bubbles are forcibly passed at a designated period of 0.1 to 10 sec. The length of the bubble can be controlled to be equal to 30, 20 and 10 mm in length. The experiment shows that the heat transfer coefficients at the subcooling of 40 and 70 K are determined only by the period of the passing bubble within the experimental range. This result means that the enhancement of the heat transfer due to the passing bubble is mainly made by sweeping away the thermal layer on the heated surface with its bubble.