Atomic layer doping ofp‐type carbon impurity in GaAs was demonstrated using flow‐rate modulation epitaxy. An extremely narrow capacitance‐voltage profile with 5.8 nm full width at half‐maximum is observed in the wafer with a sheet hole density of 9.5×1011cm−2. Atomic layer doping of carbon was performed by supplying trimethylgallium or trimethylaluminium instead of triethylgallium. It was found that the sheet hole density does not change before and after annealing for 1 h at 800 °C indicating that the carbon is a very stable impurity in GaAs. The diffusion coefficient of carbon is estimated to be 2×10−16cm−2/s at 800 °C. This is the lowest value ever reported forp‐type impurities.