We have investigated relationships between microscopic structure and cathodoluminescence (CL) property of polycrystalline (poly-) GaN grown by electron-cyclotron-resonance plasma-excited molecular beam epitaxy (ECR-MBE) on ZnO/Si substrates. Very strong CL with a peak of 3.45 eV was observed from the poly-GaN, which mainly showed a columnar structure with a size of 50-100 nm. On the other hand, the intensity of CL from the poly-GaN with few columnar domains was weaker than that of the poly-GaN with the columnar structure. The CL image from the poly-GaN, in which the columnar domains were locally observed, showed a strong contrast between bright domains and a dark background. It is confirmed that these bright regions in the CL images are corresponding to the columnar domains of the poly-GaN, by comparing with the SEM images. These results suggest that the columnar domains are responsible for the strong CL from the poly-GaN grown on the ZnO/Si substrates. Cross-sectional transmission electron microscope (TEM) observation revealed that the columnar domains had high quality crystallinity with few defects.