  We report time-resolved spectroscopic observations of the SU Ursae Majoris
dwarf nova, YZ Cnc, for 2 nights over 11 hrs during its 2002 January
superoutburst. The spectra only show absorption-line profiles in the first day.
But the lines display blue and red troughs, with ``W'' profiles in the second
day. The radial velocity curve of the absorption troughs and emission peaks of
H$\beta$ has an amplitude of $49\pm10$ km s$^{-1}$ and a phase offset of
$-0.07\pm0.04$, which are very similar to those measured in quiescence;
however, the $\gamma$ velocity deviates strongly from the systemic velocity
measured in quiescence, showing variation of the order of $\pm$60 km s$^{-1}$.
And large shifts of $\sim$70 km s$^{-1}$ and $\sim$0.09, for the
orbital-averaged velocity and phase respectively, are also found in our
observations. All these phenomena can be well explained with a precession of an
eccentric disk and we conclude that these phenomena are the characteristic
products of an eccentric accretion disk.
