  It is shown that Hawking radiation of Dirac particles does not exist for
$P_1, Q_2$ components but for $P_2, Q_1$ components in a charged Vaidya - de
Sitter black hole. Both the location and the temperature of the event horizon
change with time. The thermal radiation spectrum of Dirac particles is the same
as that of Klein-Gordon particles. Our result demonstrates that there is no new
quantum effect in the thermal radiation of Dirac particles in any spherically
symmetry black holes.
