Interactions between ferric species and silica have been shown to play a large part in the coagulation of suspensions. These effects are quite distinct from those associated with polymeric or colloid silica. The interaction of monosilicic acid with Fe ions has been discussed in this paper. Compared with traditional iron salt, poly-silica-ferric-chloride (PFSC) is characterized by having higher molecular weight, less positive charge, lower Feb and higher Fec. In addition, the Si/Fe is higher and the Silica is higher and Silicc is lower. With increasing Si/Fe ratio the effective diameter of PFSC increases. Along with the increase of Si/Fe ratio, more complicated polymers are formed and the particle size distribution is broadened. Resluts show that some co-polymerized species or aggregates between silica and Fe( ) polymers are formed.