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It is well known that the surface structure affects the pool boiling heat transfer from a heater surface. The number and size distribution of cavities present on a heater surface affect the nucleation characteristics. This fact is utilized in developing structured and sintered surfaces for enhanced boiling performance. The nucleate boiling component in flow boiling is also expected to exhibit a somewhat similar dependence. The present study investigates the flow boiling heat transfer of subcooled water over heater surfaces prepared with different roughness characteristics. The setup consists of a 9.5 mm diameter circular heater placed on the lower wall of a 3 mm x 40 mm horizontal channel. Four different surface finishes are tested on the same heater unit. Each surface is observed under a microscope, and analyzed with an image processing software to obtain the cavity size distribution. Relationships between the cavity size distribution and the heat transfer are then compared for the four surfaces. The results indicate that the wall superheats for the four surfaces tested were within 0-30 percent of each other for a given heat flux. Although the roughest surface generally performed the best, no definite trend could be observed. It is also noted that the cavity size distribution of a surface is not significantly affected by simple polishing techniques. A commercially prepared sintered surface is also tested. Its performance was markedly above the four roughened surfaces. However, the sintered surface exhibited a large hysteresis effect, while the four roughened surfaces showed little hysteresis under the test conditions. NOMENCLATURE A surface area, m2 d tube diameter, m dH hydraulic diameter, m h enthalpy, J/kg hlv latent heat of vaporization, J/kg n number or cavities; n/A – cavity density pg pressure (vapor phase), Pa p l pressure (liquid), Pa q ” heat flux, W/m2 r bubble radius, m r * non-dimensional cavity radius, eq. (4) rc cavity radius,