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Summary: This work describes the theoretical and experimental investigation of an in‐house produced 63Ni radioisotope‐powered GaN‐based direct conversion (betavoltaic) nuclear battery. GaN p‐n junction device with 1‐mm2 area was fabricated and irradiated by the 63Ni 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 63Ni source is absorbed within 1 μm 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 63Ni radioactive film thickness. 63Ni film was electroplated on one face of 1‐mm2 copper plate and mounted 1 mm over the semiconductor device. A 63Ni source with an apparent activity of 0.31 mCi produced 0.1 ± 0.001 nA short‐circuit current (Isc), 0.65 V ± 0.0022 open‐circuit voltage (Voc), and 0.016 nW ± 0.0002 maximum power (Pmax) in the semiconductor device. The filling factor (FF) of the betavoltaic cell was 25%, and the conversion efficiency (ɳ) was 0.05%. Finally, experimental results were compared with theoretical calculations. [ABSTRACT FROM AUTHOR]