The characteristics of the two-phase flow heat transfer of R-134a in microtubes with inner diameters of $430;{mu}m$ and $792;{mu}m$ were experimentally investigated. The effect of the heat flux on the heat transfer coefficient for microtubes was significant before the transition quality. The boiling number expressed the interrelation between the heat flux and the mass about the heat transfer coefficients. The smaller microtube had greater heat transfer coefficients; the average heat transfer coefficient for the tube A ($D_i=430;{mu}m$) was 47.0% greater than that for the tube B ($D_i=792;{mu}m$) at $G=370=;kg/m^2{cdot}s$ and $q"=20;kW{cdot}m^2$. A new correlation for the evaporative heat transfer coefficients in microtubes was developed by considering the following factors: the laminar flow heat transfer coefficient of liquid-phase flow, the enhancement factor of the convective heat transfer, and the nucleate boiling correction factor. The correlation developed in present study predicted the experimental heat transfer coefficients within an absolute average deviation of 8.4%.