The effects of the copper content on the bendability of Al-Mg-Si alloy T4 sheets were investigated. The Al-Mg-Si alloys with less than 0.01mass%Cu, 0.4mass%Cu and 0.8mass%Cu were prepared, and the time of solution heat treatment was changed to obtain different dispersion conditions of the second phase particles and to obtain different shear band formation conditions by bending. For the samples with less than 0.01mass%Cu and 0.4mass%Cu, no cracks were observed during the bending. For the sample with 0.8mass%Cu, the maximum depth of the crack by bending increased with the time of solution heat treatment up to 75 seconds, and then decreased over 75 seconds. The second phase particles decreased by increasing the solution heat treatment time, while the formation of shear bands by bending increased by increasing the solution heat treatment time and the copper content. The cause of the occurrence and the propagation of cracks by bending are considered to be the combined effect of the shear band formation across some grains and the micro-voids formed around the second phase particles. Improving of the bendability requires a decrease in the size and number of the second phase particles and/or reduced shear band formation during the bending.