Hall measurement with Van der Pauw method and the Capacitance-Voltage(C-V) characteristics method are performed on AlGaN/GaN heterostructures with different Al contents grown on sapphire substrates by metalorganic chemical vapor deposition. It is found that the value of sheet carrier density obtained from Hall measurement is larger than that deduced from C-V carrier density profile, and both values, as well as the difference between them increase with increasing Al content. This result is ascribed to two reasons. On the one hand, Ni/Au Schottky contact deposited on AlGaN/GaN heterostructure changes the surface states of the AlGaN barrier layer. Some electrons in the two-dimensional electron gas (2DEG) are extracted to the void surface donor states, and consequently the 2DEG sheet carrier concentration is reduced. And with the Al content increasing, the more the surface states of the AlGaN layer, the more the electrons are extracted to the void surface donor states. On the other hand, the precision of C-V measurement is influenced by the series resistance, which causes underestimation of the magnitude of the depletion-layer capacitance and hence the carrier concentration.