  The influence of (non-magnetic and magnetic) impurities on the transition
temperature of a d-wave superconductor is studied anew within the framework of
BCS theory. Pairing interaction decreases linearly with the impurity
concentration. Accordingly $T_{c}$ suppression is proportional to the
(potential or exchange) scattering rate, $1/\tau$, due to impurities. The
initial slope versus $1/\tau$ is found to depend on the superconductor contrary
to Abrikosov-Gor'kov type theory. Near the critical impurity concentration
$T_{c}$ drops abruptly to zero. Because the potential scattering rate is
generally much larger than the exchange scattering rate, magnetic impurities
will also act as non-magnetic impurities as far as the $T_{c}$ decrease is
concerned. The implication for the impurity doping effect in high $T_{c}$
superconductors is also discussed.
