GaN layers with columnar structures were grown by electron-cyclotron resonance (ECR) plasma-excited molecular beam epitaxy (MBE) with hydrogen-nitrogen-mixed gas plasma. The structure and defect characteristics of the GaN layers were investigated using scanning electron microscopy and transmission electron microscopy. The columnar structures showed a crystallographic hexagonal shape with a density of 2.8 x 10(9)/cm(2) and a size of 50-200 nm. The surface of the columns had hexagonal pyramids with (1 0 (1) over bar 1) facets. A high density of stacking faults parallel to (0 0 0 1) plane was observed in the columnar domains. The relationship between the formation of stacking faults and the effect of hydrogen on ECR-MBE GaN growth was discussed.