FNAL-COLLIDER. Detailed direct measurement of the Top Quark Mass using lepton+jet events in pbar-p interactions at a centre-of-mass energy of 1.8 TeV. A value of 173.3 +- 5.6 +- 5.5 GeV is determined. Combining with the previous measurement of this mass, using dilepton events as reported in Fermilab-Pub-97/172-e (168.4 +- 12.3 +- 3.6 GeV) , gives a new best average from D0 of 172.1 +- 5.2 +- 4.9 GeV, or 172.1 +- 7.1 GeV. The cross section value is determined with this new mass of top quark. The collaboration marks that & apos;the agreement of the Standard Model expectation with our measurement is excellent& apos;.