This paper reports the experimental study of natural convection heat transfer with $Al_2O_3$-water nanofluid. Experimental apparatus was a cylindrical enclosure with adjustable fluid layer thickness, and the aspect ratio was varied between 10.9 and 30.4. Heat transfer coefficients seemed to have reached a steady value within 30 minutes as the case with pure water. But, decrease in heat transfer coefficient continued for over $1{sim}2$ hours for inclination angle of $0^{circ}$, and oscillation in heat transfer was observed for certain inclination angles and aspect ratios for over 10 hours. Oscillation shape and period depended on the aspect ratio and inclination angle. For example, the oscillation period for $0^{circ}$ was more than twice that for $60^{circ}$. The maximum Nusselt number occurred at the inclination angle of $30^{circ}$, and the minimum occurred at $60^{circ}$ for Rayleigh number less than 1.E5. However the present results were obtained with aggregated nanofluid and would be devoid of generalities.