The effects of addition of 0.01 and 0.03 wt% Mg on the bend formability and strength of a Cu-1.8 wt% Be-0.21 wt% Co alloy aged at 320 A degrees C for 30 min have been investigated metallographically. The addition of Mg to the Cu-Be-Co alloy enhances the bend formability and strength of the alloy. The enhancement of strength is caused by the increase in volume fraction of gamma(1) precipitates in the Cu matrix. In bending of the alloys with and without 0.01 and 0.03 wt% Mg, a number of micro necks first arise along grain boundaries, and part of them grows, resulting in surface wrinkles, which finally lead to surface cracking. The cracking is initiated from voids formed by destruction of bar-like gamma precipitates in discontinuous precipitation (DP) cells and propagates along grain boundaries. The addition of Mg decreases the width of DP cells, resulting in better bend formability. This arises because smaller stress concentration due to less inhomogeneous deformation develops in cells and, as a result, destruction of the gamma precipitates in cells occurs less easily as the cell width decreases.