Microstructures of a CuZnAlMnNi shape memory alloy in the as-quenched and long-term aged conditions were investigated by transmission electron microscopy. Aged for one year in martensite phase, an equilibrium (-phase with fcc structure was observed in the M18R martensite matrix, accompanied by the appearance of a novel diffraction pattern. By analysis, it was suggested that the novel pattern results from the (-phase and the martensite matrix remaining in seven fine plates which produce intense secondary diffraction effect when the diffraction beams enter from one phase into another.