Abstract: Clumpiness of intergalactic medium in sub-Mpc scales can boost the recombination rate significantly, thereby affecting the growth of HII bubbles during the Epoch of Reionization (EoR). In this study, we use high-resolution hydrodynamic simulation with radiative-transfer to assess clumpiness of ionized gas in sub-Mpc scales to provide a sub-grid preion for unresolved clumping factor for large-scale EoR simulations. Compared to previous studies, we extend the mass resolution well below 10^8 M_sun to take into account the structures that can form in pre-ionized medium with T<10^4 K. During a few megayear after the volume is exposed to the radiation, ionization fronts sweep across low density gas super-sonically until they are trapped on surfaces of minihalos. Small-scale density structures during this time lead to a large (C>10) clumping factor for ionized gas and hugely boosts the recombination rate until they are mostly disrupted by hydrodynamic feedback after ~10 Myr. When our sample volume gets ionized at z_i with a blackbody radiation with the intensity J_21, the integrated recombination from the structures is N^add_rec = 0.32* (J_21)^0.12 [(1+z_i)/11]^-1.7 per H atom on top of what is expected from homogenous distribution of the gas. In models that most the volume ionized near the end of reionization, this can add significantly (N^add_rec ~> 0.5) to the ionizing photon budget to achieve the reionization.