This paper presents the measurement of the relative width difference ΔΓ d /Γ d of the B 0 ‐ B ¯ 0 system using the data collected by the ATLAS experiment at the LHC in pp collisions at √s = 7 TeV and √s = 8 TeV and corresponding to an integrated luminosity of 25.2 fb −1 . The value of ΔΓ d /Γ d is obtained by comparing the decay-time distributions of B 0 → J / ψK S and B 0 → J / ψK *0 (892) decays. The result is ΔΓ d /Γ d = (−0.1±1.1 (stat.)± 0.9 (syst.)) × 10 −2 . Currently, this is the most precise single measurement of ΔΓ d /Γ d . It agrees with the Standard Model prediction and the measurements by other experiments.