By means of first-principles calculations, we investigate the structural and electronic properties in a series of InAs/InP-core-shell nanowires with different sizes and compositions. Analysis of the subband charge density distributions reveals that both electrons and holes are spatially localized at the InAs-region of the core-shell nanowires, in consistent with a type-I band alignment. The size and composition effects on the confinement energies of electron carriers and the band gaps in the core-shell nanowires are investigated detailedly. These results provide an important guidance for the fabrication and applications of the core-shell nanowires. (C) 2010 Elsevier B.V. All rights reserved.