  The spin-orbit coupling systems with a zero magnetic field is studied under
the equilibrium situation, {\it i.e.}, without a voltage bias. A persistent
spin current is predicted to exist under most circumstances, although the
persistent charge current and the spin accumulation are identically zero. In
particular, a two-dimensional quantum wire is investigated in detail.
Surprisingly, a persistent spin current is found to flow along the confined
direction, due to the spin precession in accompany with the particle motion.
This provides an interesting example of constant spin flowing without inducing
a spin accumulation, contrary to common intuition.
