We report on the effect of surface passivation on the electrical characteristics of multi-channel ZnO nanowire field-effect transistors (FETs). Surface passivation was performed using a SiO2 layer on ZnO nanowires. Multi-channel FETs were prepared by assembling as synthesized ZnO nanowires on a SiO2/Si substrate using an alternating current (AC) dielectrophoresis (DEP) technique. We observed that surface passivation with a SiO2 layer on ZnO nanowires was significantly affected by electrical characteristics of multi-channel ZnO nanowire FETs such as the threshold voltage and transconductance.