  Diffusion effect on chirality selection in a two-dimensional
reaction-diffusion model is studied by the Monte Carlo simulation. The model
consists of achiral reactants A which turn into either of the chiral products,
R or S, in a solvent of chemically inactive vacancies V. The reaction contains
the nonlinear autocatalysis as well as recycling process, and the chiral
symmetry breaking is monitored by an enantiomeric excess $\phi$.
  Without dilution a strong nonlinear autocatalysis ensures chiral symmetry
breaking. By dilution, the chiral order $\phi$ decreases, and the racemic state
is recovered below the critical concentration $c_c$. Diffusion effectively
enhances the concentration of chiral species, and $c_c$ decreases as the
diffusion coefficient $D$ increases. The relation between $\phi$ and $c$ for a
system with a finite $D$ fits rather well to an interpolation formula between
the diffusionless(D=0) and homogeneous ($D=\infty$) limits.
