The bandgap narrowing effect of carbon doped GaAs epilayers as a function of substrate orientation, temperature and hole concentration has been investigated using photoluminescence spectroscopy. The bandgap energy of the epilayers shows a strong dependence on the substrate orientation. The dependence results from the difference in carrier concentration at each substrate orientation rather than the lattice mismatch between the carbon doped epilayer and GaAs substrate. The measured temperature dependence of the bandgap energy is well expressed by the Varshni equation. The bandgap narrowing (DeltaE(g)) as a function of hole concentration (p) is found to be DeltaE(g) = -2.5 x 10(-S) p(1/3) and compared with the previously reported results. From the measurements of hole concentration, the effective bandgap at zero temperature is determined in hole concentrations ranging from 10(18) to 10(20)cm(-3).