  We investigate leptonic decays $D_s \to \tau \nu_\tau$ and $B^+ \to \tau
\nu_\tau$ in R-parity violating (RPV) supersymmetric standard model. Taking
account of interference between the s-channel slepton exchange and the
$t$-channel squark exchange diagrams, we find that the supersymmetric
contributions are cancelled between two diagrams so that the RPV couplings
could be sizable under the experimental bounds. Constraints on the relative
sign between the RPV couplings in $s$- and $t$-channel diagrams are also
discussed.
