  We calculate charmed meson spectra and decay constants in lattice QCD
employing one-loop $O(a)$ improved heavy quark action and axial-vector
currents. In quenched simulations at $a \sim 0.1$ fm with the plaquette gauge
action as well as a renormalization-group improved one, it is shown that the
deviation from the continuum dispersion relation and the violation of
space-time symmetry for the pseudoscalar meson decay constants are
substantially reduced, once the $O(a)$ improvement is applied. Preliminary
results with two flavors of dynamical quarks are also presented.
