A solar air heater is an effective way to heat air and has applications in heating rooms, drying crops, vegetables and seasoning timber. This study conducted an experiment to evaluate the impact of different slant angles (30°, 45°, 60°) and mass flow rates (0.040 kg·s−1, 0.045 kg·s−1, 0.052 kg·s−1) of fins on the thermal efficiency of finned plate solar air heaters. A Finned Plate Solar Air Heater (FPSAH) system with an inclined fin array was developed and tested for thermal efficiency. An array of fins with different slant angles was attached to the rear of the absorber plate to improve heat transfer and thermal efficiency. Results showed that the thermal efficiency increased as the slant angle decreased, with the highest thermal efficiency (70 %) and outlet temperature (58.66 °C) achieved at a slant angle of 30° and mass flow rate of 0.052 kg·s−1. The FPSAH system uses inclined fins to increase solar energy absorption and generate high air temperatures at the outlet. It performs better thermally compared to a solar air heater with a flat plate.