The present work studied the precipitate microstructures in as-cast Mg-Zn-Y-Zr alloys. The experimental result showed that there is significant number of small precipitates within the grains besides the icosahedral quasicrystals along the grain boundaries. Among these precipitates, a new phase has been identified. The new phase displays square morphologies with the size in the range of 200 nm to 2 μm. According to the energy dispersive spectrometer (EDS), this phase could be a metallic Y-riched compound. The diffraction patterns can be indexed with an f.c.c. structure with the lattice parameter a = 0.52±0.1 nm. The structure does not agree with any precipitate structures that have been reported from the previous studies of Mg-Zn-Y alloys.