We have investigated relationships between the microscopic structure and optical property of polycrystalline GaN grown by (electron-cyclotron-resonance plasma-excited molecular beam epitaxy) ECR-MBE on silica glass substrates, using scanning electron microscope (SEM) and cathodoluminescence (CL). It was found that CL intensity was stronger for the samples with a large columnar domain size. These individual columnar domains showed clear luminescence. It was found that the origin of strong luminescence from polycrystalline GaN is due to such a columnar domain. That luminescence was closely related to the morphology of the columnar domains. It was revealed that the columnar domain with a homogeneous and hexagonal shape showed clear luminescence.