The precipitates responsible for the age-hardening effect in Cu-1.11%Mn-0.28%P alloy have been studied using transmission electron microscopy (TEM). The precipitates in the aged alloy have a rod shape and a composition of Mn2P having a hexagonal structure. Their axial direction is [0001](p) and is parallel to < 001 >(m) of copper matrix, where subscripts 'm' and 'p' signify matrix and precipitate, respectively. Orientation relationships between the precipitates and the matrix are as follows: [001] (m) // [0001] (p) // rod axis and (010) (m) // (13 (4) over bar 0) (p) [001] (m) // [0001] (p) // rod axis and (010) (m) // (12 (3) over bar 0)(p)