Effect of microstructure on micro-cracking behavior of Al-Mg-Si alloy extrusions during axial compressive deformation was studied. Extrusions of Al-Mg-Si alloys with two different compositions were used for the mechanical tests and microstructure observation. Compressive loads were applied parallel to the axes of the tubular specimens with rectangular cross section. As deformation proceeded, the specimens changed their shape into bellows-like ones. Specimens with finer grains showed higher critical strength and strain for crack initiation. Microscopic observation showed that cracks initiated at the bulge surface of the bellows and propagated into the depth. The observed behavior of crack initiation and propagation was interpreted in connection with those in simple tensile tests. The stress and strain at the crack initiation site of the bulge were assumed to correspond to the tensile strength of the alloy. The effect of grain size on the crack behavior was well explained by the grain size dependence of stress concentration at grain boundaries due to dislocation pile-ups. The effects of over-aging on the microstructure and crack initiation behavior were also discussed.