We present {it Chandra} HETG observations of SS Cygni in quiescence and outburst. The spectra are characterized by He-like and H-like Ka emission lines from O to Fe, as well as L-shell emission lines from Fe. In quiescence, the spectra are dominated
by the H-like Ka lines, whereas in outburst the He-like lines are as intense as the H-like lines. In outburst, the H-like Ka lines from O to Si are broad, with widths of 4--14 eV in Gaussian $sigma$ (1800--2300$ {rm km s^{-1}}$). The large line widths, together with line profiles, indicate that the line-emitting plasma is associated with the Keplerian disk and still retains the azimuthal bulk motion. In quiescence, the emission lines are narrower, with a Gaussian $sigma$ of 1--3 eV (420--620$ {rm km s^{-1}}$). A slightly larger velocity for lighter elements suggests that the lines in quiescence are emitted from an ionizing plasma at the entrance of the boundary layer, where the bulk motion of the optically thick accretion disk is converted into heat due to friction. Using the line intensity ratio of He-like and H-like Ka lines for each element, we have also investigated the temperature distribution in the boundary layer both in quiescence and outburst. The distribution of SS Cyg is found to be consistent with other dwarf novae investigated systematically with {it ASCA} data.
