  We present the discovery of two distinct classes in the Swift short duration
gamma-ray bursts (S-GRBs) from the X-Ray Telescope (XRT) X-ray afterglow light
curve. We find that about 40% of the Swift S-GRBs have an X-ray afterglow light
curves which only lasts less than 10000 seconds after the burst trigger
(hereafter short-lived S-GRBs). On the other hand, another 60% of S-GRBs have a
long lasting X-ray afterglow light curve which resembles the long duration
gamma-ray bursts. We also find that none of the short-lived S-GRBs shows the
extended emission in the Burst Alert Telescope (BAT) energy range. We compare
the burst properties for both the prompt emission and the afterglow, and
discuss the possibility of different progenitors for the Swift short GRBs.
