Poiseuille flow of simple fluids in cylindrical nanochannels is investigated by nonequilibrium molecular dynamics simulations. The effects of pore shape and size are studied by comparing the fluid behavior in cylindrical and slit-like pores. It is found that the cylindrical pores are involved with a stronger molecular interaction between the fluid and wall atoms due to larger surface-to-volume ratio, curvature and possibly greater molecular roughness. The fluid-wall interaction plays an important role in the fluid flow at nanoscale. As a result, the bulk fluid density in cylindrical pores is lower than in the slit-like pores because a larger number of fluid particles are apposed onto the wall surface. In addition, evident slip on the boundary is observed in the cylindrical pore while no slip occurs in the slit-like pore. The fluid in the cylindrical pores has lower temperature and more uniform temperature distribution. This suggests that the cylindrical pores perform better to conduct the viscous heat out of the channel than the slit-like pores. (C) 2007 Elsevier Ltd. All rights reserved.