Highly ordered Co doped ZnO:Co nanorods arrays with Co concentrations of 1.6, 1.9 and 3.1 at% were uniformly grown on FTO glass substrate by hydrothermal method. The X-ray diffraction patterns of the undoped and Co doped ZnO nanorods revealed characteristic peaks of (1 00), (0 0 2), (1 0 1), (1 0 3) and (1 1 2), corresponding to the hexagonal wurtzite phase of ZnO. For ZnO:Co nanorods with Co concentrations of 3.1 at%, the NO2 response reached a maximum value of 88 at temperature of 210 degrees C. However, the response of ZnO:Co nanorods with Co concentrations of 3.1 at% decreased from 82 to 29 with the increasing of O-2 annealing temperature from 0 to 700 degrees C. As confirmed by the XPS, PL, Raman and I-V results, the oxygen vacancies and electron concentrations were the dominating effects and an oxygen vacancy mediated NO2 sensing mechanism was presented and discussed. (C) 2015 Elsevier B.V. All rights reserved.