Solar air heaters are becoming more and more popular in space heating and industrial processes due to their cost effectiveness and easy maintenance. However, how to increase thermal efficiency of solar air heaters becomes a major challenge. In this paper, the design of a solar air heater with offset strip fins was optimized by numerical modeling. Then, a series of experiments based on ASHRAE Standard 93-2003 was conducted to test the detailed thermal performance of the heater in the light of time constant, thermal efficiency, incident angle modifier and the synthetical resistance coefficient. Results indicate that the instantaneous thermal efficiency exceeded 0.40 even when the heater was running at low airflow rate (100 m(3) h(-1)) under the typical space heating application where the solar irradiance on the collecting area was 600W m(-2), indoor air temperature 14 degrees C, outdoor air temperature -5 degrees C, and solar incidence angle 20 degrees. Such a design would only require fan power of approximately 20W. The work would be useful for developing energy efficient and cost effective solar air heaters. (C) 2013 Published by Elsevier Ltd.