A flexibility-based sensitivity approach for structural damage detection is presented under ambient vibration. The most significant contribution is in the development of a simple process to solve the mass normalization problem surrounding the use of ambient vibration modes. By the transposition of the flexibility sensitivity 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 can be also obtained. The effectiveness of the proposed method is illustrated using simulated data with measurement noise on a two-storey frame structure. Results show that the proposed method can accurately identify structural damages only using the first few arbitraryscaled mode shapes. The predicted mass normalization factors of measured mode shapes by the proposed method have good accuracy. It has been shown that the proposed procedure is computationally attractive and simple to be implemented. The presented scheme is useful for structural damage identification in ambient vibration case.