The purpose of this study is identity the crystal structure of metastable phase in Ag added Al-Mg-Si alloy by comparing the beta'-phases in Al-Mg-Si alloy without Ag, using images of high resolution transmission electron microscope (HRTEM), selected area electron diffraction patterns (SADPs) and an energy dispersive X-ray spectroscopy (EDS). The result of SADPs and HRTEM images have been simulated and compared with images and SADPs obtained from actual precipitates. SADPs and HRTEM images obtained from metastable phase in the Ag added Al-Mg-Si alloy showed similar to those of beta'-phase in Al-Mg-Si alloy without Ag and had different lattice spacings because of the effect of Ag.