The warping deformation along grain boundaries has been found for thin plates of Cu-1.8 mass%Be-0.2 mass%Co and Cu-1.8 mass%Be-0.2 mass%Co-0.03 mass%Mg alloys aged at 593 K for 3 h after cold rolling to 0% and 20% reduction, but not been detected for the two alloys aged at the same condition after cold rolling to 90% reduction, by surface roughness measurements. The aging produces γ′ precipitates in grains and discontinuous precipitation (DP) cells at grain boundaries in the two alloys after 0% and 20% cold-rolling, but no DP cells in the alloys after 90% cold-rolling. The addition of Mg or the increase in cold-rolling rate decreases the width of DP cells, resulting in reduction in the degree of warping deformation. The warping deformation along grain boundaries can be attributed to the difference between the length changes of the DP cell and the grain interior by the aging.