The purpose of this work is to describe the two- phase flow structure and heat transfer of unsteady natural convective boiling in a narrow vertical channel. The experiments are performed with saturated n- pentane at a pressure of 1 bar. An unheated plate is placed parallel to the heating surface and lateral sides are closed. The distance between the heated surface and the confinement plate is 800 mu m. Void fraction measurements are performed using capacitive sensors. The void fraction increases with heat flux and reaches a maximum of 0.80 in the mid- height of the channel when the heat flux is equal to 90% of the critical heat flux. Flow observations using a high- speed video camera show an unsteady thermo- hydraulic behavior. The frequency of the cycles increases with the wall temperature during nucleate and transient boiling. Local velocities of the bubble meniscus developing within the confined space are determined during the boiling cycles. The time- averaged liquid flow rate increases significantly with heat flux and reaches a maximum for heat flux close to the critical heat flux.