Carrier concentration in heavily carbon‐doped p+‐AlGaAs epilayers grown by metalorganic chemical vapor deposition is increased by annealing at temperatures from 500 to 800 °C under a SiN capped condition in nitrogen gas ambient and is decreased by annealing at 850 °C. The carrier concentration reaches a maximum value corresponding to nearly 100% activation at about 600 °C. These thermal behaviors can be well explained by the overlapping of two components. One is the carrier increase which occurs even at as low as 500 °C and is related to the decrease of hydrogen atoms in the epilayers. The hydrogen atoms probably deactivate the carbon acceptors in a nearly one‐to‐one correspondence. The other is the carrier decrease at temperatures higher than 600 °C, which may be due to a change in the occupation site preference of carbon atoms from arsenic sites.