Tailor rolled blanks (TRB) of Al–5.5wt%Mg alloy were fabricated by flexible rolling process, the thickness reduction of thin parts did not exceed 50%. In order to optimize heat treatment condition for increasing formability of TRB sheets, annealing treatment of the sheet having different rolling rate was conducted at the temperature range of 350–450°C for 1h. The microstructure and tensile properties of the sheets were evaluated and the formability of the sheets was investigated by evaluating plastic anisotropy (R) and punch stretching test. The Al–5.5wt%Mg alloy sheets annealed at 400°C for 60min showed high strain hardening and low plastic anisotropy at the whole range of reduction rate (0–45%).