High-resolution electron microscopy has been applied to microstructural observations of an amorphous Pd_77.5Cu₆Si_16.5 alloy. In all the specimens prepared at different cooling rates, images of microcrystalline domains were clearly observed and thereby the formation of the medium range order (MRO) in this alloy was proved. The average size of these MRO domains was found to vary from 3.0 to 1.5 nm depending on the cooling rate and on the melt temperature on cooling. From the geometries of the crossed-lattice images and also from the microarea electron diffraction, the structure of the MRO domains was identified as that of palladium with the face-centered cubic structure.