This paper presents a generalized flexibility perturbation approach for structural damage detection under ambient vibration. The most significant contribution of this work is in the development of a simple process to solve the mass normalization problem surrounding the use of ambient vibration modes. By transposition of the generalized flexibility perturbation equation, the mass normalization factors of the arbitrary-scaled mode shapes along with the elemental damage parameters can be calculated simultaneously using the least-square method. As a by-product of the proposed approach, in addition to the extent of the damage, an estimate of the mass normalization factor also can be obtained. The effectiveness of the proposed method is illustrated using simulated data with measurement noise on a two-storey frame structure. The results show that the proposed method can identify structural damages accurately only using the first arbitrary-scaled mode shapes. The predicted mass normalization factor of measured mode shape by the proposed method also has good accuracy. It has been shown that the proposed procedure is computationally attractive and simple to implement. The presented scheme may be useful for structural damage identification under ambient vibration case.