We investigate undoped GaN and Mg-doped GaN grown by rf plasma-assisted molecular beam epitaxy (MBE) with different Mg concentrations by photoluminescence (PL) at low temperature, Hall-effect and XRD measure- ments.In the PL spectra of lightly Mg-doped GaN films, a low intensity near band edge (NBE) emission and strong donor-acceptor pair (DAP) emission with its phonon replicas are observed.As the Mg concentration is increased, the DAP and NBE bands become weaker and a red shift of these bands is observed in the PL spectra.Yellow luminescence (YL) is observed in heavily Mg-doped GaN.The x-ray diffraction is employed to study the structure of the films.Hall measurement shows that there is a maximum value (3.9 × 10 cm) of hole concentration with increasing Mg source temperature for compensation effect.PL spectra of undoped GaN are also studied under N-rich and Ga-rich growth conditions.Yellow luminescences of undoped Ga-rich GaN and heavily Mg-doped GaN are compared, indicating the different origins of the YL bands.