Experimental results are presented on natural convection heat transfer from a vertical cylindrical heater to mercury, sodium-potassium eutectic, potassium and lithium in the presence of a horizontal magnetic field. Data of mercury are in good agreement with analytical solutions under a low magnetic intensity, and the decrease of heat transfer coefficient gradually levels off with the increase of magnetic intensity. The heat transfer coefficients of alkali metals, on the contrary, do not always decrease with the increase of magnetic intensity, but become larger than the non-magnetic field and take a maximum value at a certain low magnetic intensity at fixed heat flux. By taking a correlation method of liquid temperature fluctuations at two positions near the heater surface, it is shown that the flow changes its direction at random and the liquid velocity takes a maximum at a certain low magnetic intensity where the heat transfer coefficient takes a maximum value.