In this letter, the authors directly observed the zinc-blende (ZB) ZnO core in the initial formation of wurtzite (WZ) ZnO tetrapods. The formation of the wurtzite (0113) twined nanowires is proposed based on the ZB core. Simple bonding density calculation shows that the wurtzite nanowires with {0110} side surfaces have the lowest surface energy. A favorable choice of WZ phase over ZB when forming nanostructures is likely to be a result of surface energy minimization. This could be the reason that ZnS nanowires take WZ rather than ZB. (c) 2007 American Institute of Physics.