  We present results for form factors of semileptonic decays of $D$ and $B$
mesons in 2+1 flavor lattice QCD using the MILC gauge configurations. With an
improved staggered action for light quarks, we successfully reduce the
systematic error from the chiral extrapolation. The results for $D$ decays are
in agreement with experimental ones. The results for B decays are preliminary.
Combining our results with experimental branching ratios, we then obtain the
CKM matrix elements $|V_{cd}|$, $|V_{cs}|$, $|V_{cb}|$ and $|V_{ub}|$. We also
check CKM unitarity, for the first time, using only lattice QCD as the
theoretical input.
