  In 1995, the top quark was discovered at the Fermilab Tevatron Collider by
the CDF and D0 collaborations in the ppbar collisions at sqrt(s)=1.8 TeV using
about 50pb^-1 of data per experiment. We present the studies of the top quark
properties based on 1fb^-1 of data collected by the D0 experiment at a center
of mass energy of 1.96TeV. The increased statistics and higher collision energy
allow to perform precision measurements of the top quark production and decay
characteristics and open the possibility to probe physics beyond the Standard
Model in the top quark sector. The presentation mainly focuses on the
measurement of the top pair production cross section and branching fraction of
the top quark decays, a search for ttbar resonances, and the measurement of the
top quark pair production cross section ratio and its interpretation in terms
of new physics.
