Airborne molecular contaminants (AMCs) can affect device characteristics in semiconductor processes. Understanding the relationship between gas phase concentration and wafer surface concentration is helpful for AMCs control requirements. The aim of this study is to create the adsorption model of organic compounds on silicon (Si) wafer surfaces in cleanroom ambient for estimating the surface concentration. The molecular deposition velocity in unidirectional flow cleanroom for predicting deposition was developed by the effect of the molecular diffusion and the eddy diffusion. Then the adsorption model was composed of the Langmuir adsorption theory including the molecular deposition velocity, the sticking probability, the desorption rate, the saturated surface concentration and the volume concentration in cleanroom. This model was applied to predict the dioctyl phthalate (DOP) concentration on the Si wafer surface.