To quantitatively evaluate OBT production after a nighttime wet deposition of atmospheric HTO onto land surface, numerical experiments using a sophisticated tritium transport model (SOLVEG-II) were conducted. For a nighttime rain actually observed at a grassland site, two hypothetical scenarios for wet deposition of HTO were made; 0.1-folded and 10-folded case, each having rain HTO concentration at 0.1 folds and 10 folds of equilibrium concentration of ground-level air HTO concentration. Calculation results showed atmospheric HTO concentration after the rain at 10-folded case was more than an order of magnitude larger than that at 0.1-folded case, affected by interception and evaporation of rain HTO with leaves, and, by a larger HTO re-emission from soil. After the rain, due to these heightened atmospheric HTO, TFWT concentration in leaf cellular water at 10-folded case kept an order, or more, larger than that at 0.1-folded case. As a result, OBT produced in leaves over nine-day after the rain at 10-folded case was 18 times larger than that at 0.1-folded case. We therefore concluded that nighttime wet deposition of HTO pronouncedly increases OBT formation if rain HTO concentration exceeds equilibrium concentration for the air HTO near the ground.