Saturated nucleate boiling heat transfer and critical heat flux (CHF) were experimentally studied in a confined space which consisted of two horizontal surfaces with a lower heated surface and an upper mesh screen. The nucleate boiling heat transfer was much enhanced in such a confined space, because the mesh screen kept primary vapor bubbles forming coalescence bubble within the confined space in lower heat flux region and allowed the vapor bubble to easily escape from the confined space in higher heat flux region. Observation by a high-speed camera showed that the behaviors of vapor mushroom over the mesh screen were not significantly different from that in the unconfined pool boiling at higher heat flux. The peripheral conditions of the confined space had no obvious effects on the heat transfer characteristics. The experimental CHF data showed a good agreement with the theoretical predictions based on the microlayer model proposed by some of the present authors.