Al-Mg-Si-Cu series alloy sheet, which can be strengthened by heat treatment, is a perfect material for automotive body since it can be substituted from the conventional steel sheet for automotive body to reduce the vehicle weight and respond to the demands of energy saving and emissions reduction. However, because the crystal structure of aluminum alloy sheet is different from that of the steel sheet and especially there are a great deal of second-phases in aluminum alloy matrix, the forming characteristics of aluminum alloy sheet such as strain hardening behavior is different from that of steel sheet. Therefore the experiences obtained during forming steel sheet are not applicable to forming aluminum alloy sheet. It is urgent to clarify the relationship between microstructure especially second-phases and forming behavior of aluminum alloy sheet, from which a reasonable evaluation system of formability for aluminum alloy sheet can be established. In the present study, the variation of strain hardening exponent $n$ during plastic deformation and the strain hardening behaviors of Al-0.9Mg-1.0Si-0.7Cu-0.6Mn alloy sheet subjected to solid solution, T4 and annealing treatment were investigated by tensile test and TEM/EDS. The results show that the relationship between true strain and true stress of alloy sheet during tensile test does not fully meet the Hollomon formula, and the $n$ value of alloy sheet varies as the strain range to calculate the n value changes. The annealed alloy sheet has the most significant  strain hardening effect at the initial stage of plastic deformation, but the alloy sheet subjected to solid solution treatment shows the most distinct strain hardening effect at the latter stage of plastic deformation. The n values of alloy sheet subjected to solid solution treatment are larger than those of T4 alloy sheet at each given strain range during tensile test, which is attributed to the much lower shear stress for dislocation slip in the former sheet than in the latter sheet.