Passive heat transfer augmentation techniques, where inserts are employed in the flow passage to enhance the heat transfer rate, are advantageous as compared to active techniques because the insert manufacturing process is simple and these techniques can be easily used in an existing heat exchanger. An experimental investigation has been done by using louvered rectangular leaf type inserts by inserting it inside the horizontal circular tube. These louvered rectangular leaf type inserts were made up of aluminum material at different angles i.e. 30°, 45°, 60°, 90° respectively. Air was used as a working fluid which was passed through the circular pipe. Heat transfer rate was investigated using these louvered rectangular leaf type inserts into a plain tube. The results were further compared with those of plain tube to analyse the improvement of the heat transfer rate in presence of these inserts. Reynolds number considered for experimentation is in the range of 6000–11,000. After experimentation, it has been observed that the maximum experimental value of ‘h’ is 37.2859 W/m2K for 90° leaf angle. Also, the maximum experimental value of the Nusselt Number is 33.7478. For horizontal tube containing louvered rectangular leaf type inserts inclined at 90°, the heat transfer rate was increased by 3.9%, 16.19%, 85.28% and 70.81% for the velocity reading of 4.3 m/s, 5.6 m/s, 6.3 m/s and 7.3 m/s respectively as compared to that of plain tube.