Three-dimensional numerical simulations for the flow condensation of R-134a are performed at a saturation temperature of 40 °C inside a horizontal smooth and structured tubes with hemispherical structures on the inner surface having square pitch (2.5 mm, 4 mm, and 5 mm). The numerical model for the condensation has been validated by comparing the numerical average heat transfer coefficient with the empirical correlation for smooth tubes. Liquid-vapor interfaces at different axial and longitudinal locations for both smooth and structured tubes are presented to better understand the condensation process. The temporal and spatial averaged vapor quality and liquid film thickness for the smooth and structured tube are also shown. The results indicate that the vapor quality and liquid film thickness inside structured tube are lower than the same observed inside smooth tube. Also, the average heat transfer coefficient and pressure drop for both smooth and enhanced tubes are reported and interestingly, the tube with a 2.5 mm square pitch has shown twice the heat transfer coefficient than the smooth tube.