An experimental study on the flow boiling heat transfer in a horizontal annular passage outside the single tube using R410A. The tested tubes contain a smooth tube, a 1EHT tube (dimpled tube) and a herringbone micro-fin tube with the same outside diameter of 12.70 mm. Tests were carried out at a saturation temperature of 6 °C for a mass flux range of 8∼107 kg/m2s with a fixed inlet quality of 0.1 and three different outlet qualities (0.4, 0.6, 0.8). Changes in vapor quality and annular gap size are found to have a significant impact on boiling heat transfer in the concentric annulus. For tests in the annuli with a 25.4-mm-ID outer tube, the HTC of the herringbone micro-fin tube is highest together with the largest pressure drop. Both the annulus of 1EHT tube and the annulus of herringbone micro-fin tube show a higher boiling HTC at an outlet quality of 0.6. The larger penalty factor is found at an outlet quality of 0.8. For flow boiling in the annuli having different annular gap sizes, it is found that the heat transfer enhancement ratio decreases sharply with the increasing average quality. When the inner diameter of outer tube is 19.0 mm, HTC decreases at first and then rises slowly. The huge bubbles occurred at the low mass fluxes and the scouring effect on the heated annulus walls of high-speed flow may be responsible.