A number of steady-state numerical simulations of condensation of R134a at mass fluxes of 400kg m(-2) s(-1) and 800kg m(-2) s(-1) inside a 1-mm square cross section minichannel are proposed here and compared against simulations in a circular cross section channel with the same hydraulic diameter. The volume of fluid (VOF) method is used to track the vapor-liquid interface, and the effects of interfacial shear stress, surface tension, and gravity are taken into account. A uniform wall temperature is fixed as a boundary condition. At both mass velocities the liquid film and the vapor core are treated as turbulent; a low-Re form of the SST k- model has been used for the modeling of turbulence through both the liquid and vapor phases. Numerical simulations are validated against experimental data. The influence of the surface tension on the shape of the vapor-liquid interface may provide some heat transfer enhancement in a square cross section minichannel, but this depends on the mass flux and it may be not significant at high mass velocity, as confirmed by experimental data and by the present numerical work. The gravity force is shown to be responsible for the liquid film thickness increase at the bottom of the channel in the circular cross section, but the gravity force has a minor effect in the square minichannel; at these mass velocities, the heat transfer mechanism is dominated by shear stress and surface tension.