An experimental study is conducted to investigate the effect of hydraulic diameter on the saturated flow boiling characteristics of deionized water in parallel rectangular microchannels. Experiments have been performed for the mass fluxes of 51, 65, 78 and 93kgm(-2)s(-1), and the hydraulic diameters of 100, 150, 200 and 250m. The wall heat flux ranges from 35.9 to 105.6kWm(-2). To eliminate the effect of aspect ratio and to address clearly the effect of hydraulic diameter, all the channels are designed with square cross section. Flow visualization studies are performed for a better understanding of the underlying physical phenomena. Effects of heat flux, mass flux and vapor quality on the heat transfer and total pressure drop have been investigated, too. It is concluded that hydraulic diameter has significant influence on both of the local two phase heat transfer coefficient and the total pressure drop.