We investigate the rotational symmetry and absolute sign of the effective second-order susceptibility of the righthanded z-cut α-quartz crystal in different crystal orientations through the second-harmonic-generation phase interference between standard and reference of thin α-quartz plates. The Ds rotational symmetry of the z-cut α-quartz crystal shows 6 alteating sign sections of the second-order susceptibility in the 360° rotation, which leads to two distinctive interference pattes between the reference and standard second harmonic field. From this information, the sign of the interference patte in the second harmonic phase measurement could be readily derived.