An experimental investigation of boiling heat transfer of saturated R11 has been conducted with a horizontal cylinder which is 20 mm in diameter and 50 mm in length, and is enclosed by concentric outer tubes. These outer tubes have two holes which are 3 mm in width and 60 mm long. The clearance (c) between these tubes ranged from 0.15 mm to ∞ and heat flux (q) from 600 to 1.5×10^5 W/m^2. Heat transfer characteristics are demonstrated and discussed in this report. The main results obtained are summarized as follows: (1) The heat transfer coefficient a increases with decreasing c in the ranges of c=0.35 mm to 0.75 mm and q=0.4×10^4 to 5×10^4 W/m^2. (2) The critical heat flux q_c decreases with decreasing c when c≦0.75 mm.