  We present chromospheric activity index $S\rm_{HK}$ measurements for over
13,000 F, G and K disk stars with high signal-to-noise ratio ($>$ 60) spectra
in the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) spectroscopic
sample. A parameter $\delta$S is defined as the difference between $S\rm_{HK}$
and a `zero' emission line fitted by several of the most inactive stars. The
$S\rm_{HK}$ indices of subgiant stars tend to be much lower than dwarfs, which
provide a way to distinguish dwarfs and giants with relatively low resolution
spectra. Cooler stars are generally more active and display a larger scatter
than hotter stars. Stars associated with the thick disk are in general less
active than those of the thin disk. The fraction of K dwarfs that are active
drops with vertical distance from the Galactic plane. Metallicity affects
$S\rm_{HK}$ measurements differently among F, G and K dwarfs in this sample.
Using the open clusters NGC 2420, M67 and NGC6791 as calibrations, ages of most
field stars in this SDSS sample range from 3-8 Gyr.
