A one-frame projection algorithm is proposed to demodulate the phase by the principal component analysis (PCA).Sinusoidal fringe patterns generated by computer are projected onto the object surface by a digital-light-process ( DLP) projector and the deformed fringe patterns are acquired by a CCD camera.Firstly, five frames of phase-shifted fringe patterns are constructed from the ori ginal one-frame spatial carrier pattern.Secondly,the multiple frames of fringe patterns are decomposed into a set of values of uncorrelated variables called as pricipal components,thus the first and second principal components are obtain ed. Then,the phase is calculated from the actangent function of the two principal components.Finally,the correct global phase sign is determined by clustering method considering fringe phase monotonicity.The experimental results show that the proposed method can directly extract the phase distribution phase maps of the object without determining phase shifts.The algorithm is fast and accurate,which is suitable for on-line three-dimensional (3D) shape measuremen t.