Natural convective flows of water around large horizontal cylinders were investigated experimentally. The main concerns were the turbulent transition and its influence an the local heat transfer. The cylinders were heated with uniform heat flux and their diameters were varied from 60 to 800 mm to enable experiments over a wide range of modified Rayleigh numbers, Ra-D* = 3.0 x 10(8) to 3.6 x 10(13). The flow fields around the cylinders and the surface temperatures of the cylinders were visualized to investigate the turbulent transition mechanisms. The results show that three-dimensional flow separations occur first at the trailing edge of the cylinder when Ra-D* beyond 2.1 x 10(9), and the separation points shift upstream with increasing the Rayleigh numbers. These separations become a trigger to the turbulent transition. Transitional and turbulent flows appear downstream of the separations at higher Rayleigh numbers. However, they occupy a relatively small portion of the cylinder surfaces even at the maximum Rayleigh numbers of the present experiments. The local heat transfer coefficients were also measured. The results show that the coefficients are increased markedly in the transitional and turbulent regions.