The synthesis of ultra-long high-quality ZnO nanowires with uniform size and orientation on heterogeneous substrates is highly desirable, not only for investigating the fundamental properties of ZnO nanowires, but also for fabricating integrated functional nanodevices. Here we present a novel and general technique for growing vertically aligned ultra-long ZnO nanowires on various substrates. More importantly, the metal catalyst is experimentally determined not at the tip ends of the nanowires but at the junction area between the nanowires and the underlying substrate. Based on detailed analysis and control group results, we then propose a three-stage growth mechanism, in which vapor-liquid-solid growth and vapor-solid growth compete with each other to become dominant.