The core-annular flow has been considered as a useful tool to effectively transport highly viscous oil by locating a lower viscous fluid such as water near the pipe surface. In the present study, we simulate turbulent core-annular flows in a horizontal pipe using a level-set method to track the phase interface between oil and water. Several different superficial velocity ratios of water to oil (jw/jo) are considered, where the density and viscosity of oil are 889 kg/m3 and 0.919 Pa s, respectively. Due to the density difference of oil and water, the oil core rises, and thus the gaps between the phase interface and the wall are narrow and wide, respectively, near upper and lower pipe surfaces. With increasing jw/jo, the gap near the lower pipe surface increases more rapidly than that of the upper one. Due to this asymmetric distribution of the gap in the azimuthal direction, the flow statistics such as the wall friction and mean velocity change in that direction. For the wall friction, it is low near the lower pipe surface for all cases considered, but high near the upper (low jw/jo) and side (high jw/jo) pipe surfaces.