Abstract: In order to study the difference of flow and heat transfer characteristics under natural and forced circulations, the experiments on flow and heat transfer under natural and forced circulations were conducted at a narrow rectangular channel with one side heated for research object, and the subcooling from 30 ℃ to 60 ℃ with the constant pressure of 0.2 MPa. Under the condition of the same heat flux, the experiment on flow resistance shows that resistance characteristics in laminar regime under natural circulation are almost similar to those of forced circulation, and flow resistance in turbulent regime under natural and forced circulations can be predicted by the corrected Blasius formula. It is found that the Re_cr and the Re regions of transitional section keep the same under forced and natural circulations. The experiment on heat transfer shows that Gneilinski formula adjusted to forced circulation can forecast heat transfer characteristics in turbulent regime under natural circulation. It is found that heat transfer characteristics under forced and natural circulations have nearly no difference within the scope of this study.