  We study leptonic decays D_s\to \tau \nu and B^+ \to \tau \nu in R-parity
violating (RPV) supersymmetric standard model. The interference between the
s-channel slepton exchange and the t-channel squark exchange diagrams could be
destructive in a certain model parameter region so that sizable RPV couplings
are allowed. Contributions from the RPV interactions to the final states with
the lepton flavor violation (LFV), \tau\nu_\mu (\tau \nu_e), are also examined
taking account of constraints on the RPV couplings from the other LFV
processes.
