The relative stability between the wurtzite and zinc blende structures in GaAs nanowires with {1 1 1}/{1 (1) over bar 0 0} facets and those of {1 (1) over bar 0} / {1 1 (2) over bar 0} facets is systematically investigated using our empirical interatomic potential calculations and first-principles calculations. Our calculations clarify that the wurtzite structure is stabilized over entire diameters range for nanowires with {1 1 1}/{1 (1) over bar 0 0} facets. In contrast, for nanowires consisting of {1 (1) over bar 0}/{1 (1) over bar 0 0} facets, the zinc blende structure is favorable for diameters larger than 29 nm. This is because the surface energy difference between {1 (1) over bar 0} and {1 1 (2) over bar 0} surfaces is quite small compared to that between {1 1 1} and {1 (1) over bar 0 0} surfaces. The calculated results imply that the stability of nanowire side facets is a quite important factor determining the crystal structure. (C) 2010 Elsevier B.V. All rights reserved.