The effect of inclination angle on natural convection heat transfer has been studied for water layers. The range of the Raleigh number was from the subcritical value to 1.4${ imes}$10$^{7}$ , and the range of the inclination angle, $ heta$, measured from the horizontal was 0$leq$$ heta$$leq$180$^{circ}$. For horizontal water layers, present results agreed well with the results of previous investigators and also showed significant departures from the results of air layers in the turbulent regime. Inclined cylindrical water layers showed secondary maxima in heat transfer, whereas rectangular air layers showed continuous decline of Nusselt number.