Precipitation behavior of AlMgCu alloy during S phase aging sequence was studied with high resolution transmission electron microscope (HRTEM) and selected area diffraction (SAED) to determine the existence of SaEuro(3) phase and GPBII zone in the alloy. Fast Fourier transform (FFT) patterns corresponding to GPB zones at different aging stages were observed, and it is revealed that S phase forms instantly after the GPB zone dissolves during aging. Both simulated HRTEM and SAED images illustrate that the SaEuro(3) phase shows exactly the same characteristics as the S phase does, which is consistent with the experimental observations. A unique phase was also observed, which produced extra diffraction dots near the {012}(Al) positions in FFT pattern. This phase was once considered to be either SaEuro(3) phase or GPBII zone, but none of them has been affirmed so far. In this work, it is found that the unique phase is a distorted and rotated S phase and that SaEuro(3) phase or GPBII zone does not form during the aging. We further show that the S precipitation sequence of the aged AlMgCu alloy should be SSS -> Cu-Mg clusters/GPB zone -> S'/S(Al2CuMg).