This paper presents a set of experimental and numerical results on the Rayleigh-Benard convection of cold water near its density maximum in vertical cylindrical containers with different aspect ratio that is defined as the ratio of the diameter to the height of the container. The average Nusselt number on the bottom hot wall is measured and the effects of the density inversion parameter, the aspect ratio and the Rayleigh number are discussed in detail. Results indicate that there exists different local Nusselt number distributions corresponding to various flow patterns and the average Nusselt number decreases with the increase of the density inversion parameter. The negative effect of the density inversion parameter on heat transfer becomes more significant at high Rayleigh number values. The increasing aspect ratio can enhance the heat transfer of the Rayleigh-Benard convection. However, the enhancement effect decreases gradually with the increase of the aspect ratio. Furthermore, at large aspect ratios, multiple roll flow patterns appear, the number of the rolls increasing with the aspect ratio. Based on the experimental data, an empirical correlation on heat transfer ability is obtained within an acceptable uncertainty. (C) 2016 Elsevier Ltd. All rights reserved.