We present a combination of measurements of the top quark mass by the D0 experiment in the lepton+jets and dilepton channels. We use all the data collected in Run I (1992–1996) at √s=1.8 TeV and Run II (2001–2011) at √s = 1.96 TeV of the Tevatron $p\bar{p}$ collider, corresponding to integrated luminosities of 0.1 fb$^{−1}$ and 9.7 fb$^{−1}$, respectively. The combined result is: $m_t$ = 174.95±0.40(stat)±0.64(syst) GeV = 174.95±0.75 GeV.