  Decay constants of $P$-wave mesons are computed in the framework of
instantaneous Bethe-Salpeter method (Salpeter method). By analyzing the parity
and possible charge conjugation parity, we give the relativistic configurations
of wave functions with definite parity and possible charge conjugation parity.
With these wave functions as input, the full Salpeter equations for different
$P$-wave states are solved, and the mass spectra as well as the numerical
values of wave functions are obtained. Finally we compute the leptonic decay
constants of heavy-heavy and heavy-light $^3P_0$, $^3P_1$ and $^1P_1$ states.
