Adiabatic two-phase flow experiments have been conducted to investigate the effects of channel diameter and liquid properties on void fraction in horizontal microchannels. Water/nitrogen gas and ethanol-water/nitrogen gas mixtures were pumped through circular microchannels of 50, 75, 100, and 251 mum diameter. The ethanol concentration in water was varied to change the surface tension and liquid viscosity. The void fraction data were obtained by an image analysis technique and correlated as a function of homogeneous void fraction. The void fraction data obtained in 50, 75, and 100 mum channels confirmed well to the correlation in Kawahara et al. [1], but the data for a 251 mum diameter channel agreed with the Armand [2] correlation suitable for minichannels. There was no significant effect of liquid properties on void fraction for all channel sizes. These results suggest that the boundary between microchannels and minichannels would lie between 100 and 251 mum.