A TEM investigation on the orientation relationship of a-Co precipitates in a copper matrix has revealed that these f.c.c precipitates, approximately 15-20 nm in diameter, were formed in Cu70Co30 ribbons during melt-spinning. After annealing at 573 K, these precipitates grew to about 30-50 nm in diameter. Analysis of selected area diffraction patterns (SAPD) combined with the dark field imaging technique confirms that the orientation relationship between these a-Co precipitates and the Cu matrix is not a cube-cube relationship, but a twin-type one, i.e., {111}(Co)//{111}(Cu) and < 110 >(Co)//< 110 >(Cu).