The interaction between colloidal particles in a solution containing polymer molecules is examined theoretically. In particular, the sticking probability of colloidal particles, which plays a significant role in the determination of the collision efficiency of an unstable dispersed system, is analyzed. We found that the sticking probability is extremely sensitive to the variation in the Hamaker constant of a colloidal particle. In the conventional analysis of the collision efficiency in polymer-induced flocculation, it is assumed that the adsorption of polymer is relatively fast than the collision of colloidal particles. Since the adsorption of polymer will significantly affect the properties of the surface of a colloidal particle, this assumption may lead to an appreciable deviation in the prediction of collision efficiency.