The C-plane monocrystalline GaN/ZnO thin film epitaxially grows on A-plane sapphire substrate by molecular beam epitaxy (MBE) is revealed by the reflection high-energy electron diffraction (RHEED). The monocrystalline GaN nanomembranes have been rolled up into tubular microstructures as whispering gallery microcavities, which support the whisper gallery modes (WGMs) in the violet and blue regime. The WGMs of the rolled-up monocrystalline GaN devices are tunable with microtube diameter by tuning the strain gradient of the GaN nanomembranes. This approach could help with not only the further development of GaN-based photonic devices, but also the physical understanding of other rolled-up optical microcavities. (C) 2017 Elsevier B.V. All rights reserved.