The effect of heater diameter on the pool and flow boiling heat transfer of Freon-113 was studied. The heaters were horizontal copper tubes of diameter 1.59-7.94 mm (pool) and 3.18-12.7 mm (flow), heated internally by condensing steam. The test fluid at atmospheric pressure flowed upward, normal to the heaters, at approach velocities of 2.4, 4.0, and 6.0 m/s. For flow boiling, at a fixed velocity and wall temperature, smaller tubes resulted in higher heat flux values, except at low wall temperatures. The maximum heat flow was proportional to the 0.44 power of velocity and inversely proportional to the 0.28 power of diameter. In pool boiling the critical diameter was different for different regimes of boiling and depended on the characteristic bubble size. For tubes smaller than this critical diameter, the heat flux increased as the tube size decreased for a fixed wall temperature. Tubes larger than the critical diameter showed no diameter effect on the heat flux, except for transition boiling. For transition boiling, the heat flux increased as the tube diameter increased beyond the critical diameter.