The forced convective of heat transfer in the horizontal circular pipe without and with using the different angles of circular ribs (60°, 90° and 120°) were studied experimentally and numerically. The circular pipe was made from stainless steel of 500 mm length and 49 mm inner diameter. The circular ribs of 20 mm diameter were arranged one against the other with a distance of 100 mm between one rib and another. Air was used as a working fluid. The effect of varying Reynolds numbers (12557 to 17954) with constant surface pipe temperature of 325 K on enhancing convective heat transfer without and with using circular ribs were investigated. The experimental side included manufacturing the test section and conducting a series of experiments to show effect of varying the Reynold numbers on energy transfer and air flow characteristics. Numerical simulation included using ANSYS FLUENT (18.2) program to solve the governing equations with using Simple algorithm method and Standard k–ε turbulence model. The comparisons between the experimental results and the numerical simulation results were carried out. In general, results showed that average coefficient of the heat transfer and average Nusselt number with ribs angle of 60° were the largest compared to the other ribs angle and smooth circular pipe. The heat transfer coefficient was increased with increasing Reynolds numbers. The circular horizontal pipe with ribs angle of 60° gives a good evidence for the performance of thermal- hydraulic applications.