In this study, natural convection in solar air heaters with different fins is numerically investigated. Effect of fin shape (rectangular, elliptical and triangular), the inclination angle (45–90°) on mass flow rate, temperature and performance are studied by considering the effect of shadow created by fins. Three different cases, solar air heater with fins considering shadow effect (Shadow Effect Considered: SEC), ones with fins neglecting shadow effect (Shadow Effect Neglected: SEN), and ones without fins (WF) are compared. It can be concluded that shadow created by fins, which eliminates some part of absorber from playing a direct role in heat transfer, has a significant impact on finding the best performance and dimensions of fins. For example, the performance of rectangular and triangular fins in case SEC are about 10% and 20% less than case SEN. Due to fins shadow, mass flow rates in solar air heaters without fins (WF) is more than solar air heaters with fins in higher inclination angles. While the highest mass flow rate is achieved for depth between 0.005 and 0.010 m, but the best depth from performance point of view is about 0.025 m and improvement in performance is negligible for larger depths. •Effect of fins' shadow on solar air heaters are numerically studied.•For β > 80°, using Triangular and elliptical fins is not beneficial.•The highest mass flow rate is obtained by rectangular fins.•For β > 60°, using fins lead lower mass flow rates.•Considering the performance of solar heater, a fin depth of 25 mm is recommended.