We evaluated the possibility of monolithic integration of electron devices and surface acoustic wave (SAW) devices on GaN. We removed top n+ GaN layers of n+ GaN/unintentionally-doped GaN structures by inductively coupled plasma (ICP) etching and fabricated SAW filters on the exposed unintentionally doped GaN layers. We found that the device characteristics are almost the same as those of devices fabricated on as-grown GaN layers, although the surface morphology of GaN layers is degraded due to the ICP etching. The results indicate that SAW devices and electron devices can be monolithically integrated on GaN-based semiconductor structures.