Can Quintessence Be the Rolling Tachyon?

In the light of the recent works by Sen hep-th/0203211, hep-th/0203265, hep-th/0204143 and Gibbonshep-th/0204008, we present a phase-plane analysis on the cosmology containing a rolling tachyon field in a potential resulted from string theory. We show that there is no stable point on the phase-plane, which indicates that there is a coincidence problem if one consider tachyon as a candidate for quintessence. Furthermore, we also analyse the phase-plane of the cosmology containing a rolling tachyon field for an exactly solvable toy
potential in which the critical point is stable. Therefore, it is possible for rolling tachyon to be quintessence if one gives up the strict constraint on the potential or find a more appropriate effective potential for the tachyon from M/string theory.