The effect of a micro-structured Zirlo surface on the internal flow boiling critical heat flux (CHF) was examined under atmospheric pressure conditions for water. The Zirlo surface was modified by anodic oxidation, resulting in improved wettability with complete wetting and liquid spreading. The variation of the flow boiling CHF was investigated over a mass flux range from 300 to 1500 kg/m(2) s, for inlet temperatures of 60 degrees C, 80 degrees C, and 95 degrees C. The CHF values on a micro-structured Zirlo tube were enhanced compared to the values obtained on a bare surface, to a maximum increase of 60% at a mass flux of 1500 kg/m(2) s. However, the inlet temperature had no significant effect on the results of CHF enhancement. The CHF enhancement ratio increased with the mass flux. According to previous nanofluid experiments, the reason for flow boiling CHF enhancement should be indicated as the flow regime. Surface wettability has been invoked as one possible reason for flow boiling CHF enhancement in the nucleate boiling regime, but not in the annular flow regime. In the present study, we suggest that the flow boiling CHF in the annular flow regime increases with mass flux because of the stability of the liquid film and the liquid replenishment, resulting from both improved surface wettability and liquid spreading. (C) 2012 Elsevier Ltd. All rights reserved.