N-doped ZnO samples were grown on sapphire substrate by plasma-assisted molecular beam epitaxy (P-MBE) method. The electrical properties of the samples were investigated by temperature-dependent Hall measurement. It was found that the carrier concentration and mobility of the samples showed some unstable characteristics under different magnetic fields at a certain temperature. However, the conductivity was a much more stable parameter. By fitting the dependence of conductivity on temperature and theoretical calculating, it was believed that the conduction mechanism was possibly the mix of band and hopping conduction mechanisms. The coexistence of a huge density of donor and acceptor defects was considered to cause the instability of some electrical properties (carrier concentration and mobility) and the appearance of the hopping conduction mechanism in N-doped ZnO samples. © 2007 IOP Publishing Ltd.