This work describes the theoretical and experimental investigation of an in-house produced Ni-63 radioisotope-powered GaN-based direct conversion (betavoltaic) nuclear battery. GaN p-n junction device with 1-mm(2) area was fabricated and irradiated by the Ni-63 plate source. Short-circuit current and open-circuit voltage of the battery were measured, and current-voltage curves were plotted. The energy stored in battery, maximum power, and efficiency parameters were calculated. Monte Carlo modelling was used to investigate radioisotope's self-absorption effect, the optimization of semiconductor and source thickness, transport, and penetration of beta particles in semiconductor junction. A large fraction of beta particle energy emitted from Ni-63 source is absorbed within 1 mu m of the semiconductor junction on the basis of the simulation results. Epitaxial growth of GaN was performed using metal-organic chemical vapour deposition (MOCVD) system. Monte Carlo simulation with MCNPX was used to determine optimum Ni-63 radioactive film thickness. Ni-63 film was electroplated on one face of 1-mm(2) copper plate and mounted 1 mm over the semiconductor device. A Ni-63 source with an apparent activity of 0.31 mCi produced 0.1 +/- 0.001 nA short-circuit current (I-sc), 0.65 V +/- 0.0022 open-circuit voltage (V-oc), and 0.016 nW +/- 0.0002 maximum power (P-max) in the semiconductor device. The filling factor (FF) of the betavoltaic cell was 25%, and the conversion efficiency (& x273;) was 0.05%. Finally, experimental results were compared with theoretical calculations.