We report an investigation on the optical third-order nonlinear property of the nonpolarA-plane GaN film. The film sample with a thickness of ~2??m was grown on anr-plane sapphire substrate by metal-organic chemical vapor deposition system. By performing the Z-scan method combined with a mode-locked femtosecond Ti:sapphire laser (800?nm, 50?fs), the optical nonlinearity of the nonpolarA-plane GaN film was measured with the electric vectorEof the laser beam being polarized parallel (//) and perpendicular (?) to thecaxis of the film. The results show that both the third-order nonlinear absorption coefficient?and the nonlinear refractive indexn2of the sample film possess negative and large values, i.e.?//?=??135?±?29?cm/GW,n2//?=??(4.0?±?0.3)?×?10?3?cm2/GW and???=??234?±?29?cm/GW,n2??=??(4.9?±?0.4)?×?10?3?cm2/GW, which are much larger than those of conventionalC-plane GaN film, GaN bulk, and even the other oxide semiconductors. [ABSTRACT FROM AUTHOR]