The objective of this paper is to clarify how the heat transfer coefficient changes along the flow due to bubbles passing through a narrow vertical rectangular channel (20 mm wide, 2 mm clearance and 450 mm long). The experiments are performed using subcooled water of 80, 60 and 40 K at atmospheric pressure in which the air bubbles are injected into a channel at designated intervals from 0.125 to 1.0 sec and the bubble length is controlled to be equal to 0.03, 0.02 and 0.01 m. The experiments show that the heat transfer coefficients decrease along the flow and then reach constant values beyond a certain distance from the leading edge the heated surface where the flow reaches a fully developed state in both the velocity and the thermal conditions. Under the fully developed conditions, the heat transfer coefficients are predicted well by the existing theoretical analysis in which both the convective term and evaporation on the interface are ignored.