Experimental study on mixed convection in the entrance region of a one-side-heated narrow rectangular channel has been carried out. We have performed a series of experiments under natural circulation conditions with Re ranging from 1000 to 3000 and inclination angle ranging from 0 degrees to 30 degrees. Meanwhile, we conducted flow visualization experiments to identify secondary flow driven by temperature difference between the lower, heated and upper, unheated plate of the channel. It is found that thermal instability can be enhanced by increasing the inclination angle of the channel. The secondary flow induces the onset of thermal instability (OTI) while increasing in Re delays the OTI. A sudden increase of hear transfer coefficient and friction factor has been observed after the OTI. The traditional identification criteria for mixed convection are not suitable for the inclined, narrow rectangular channel with a heated lower side. However, transverse Richardson number can identify mixed convection in the channel. The experiment results, together with image data for the flow condition in the inclined narrow rectangular channel, offer valuable data to improve the engineering design of plate-type fuel assembly and similar heat exchangers. (C) 2017 Elsevier Ltd. All rights reserved