Poly-silicate iron coagulant (PSI) was a new coagulant for drinking water treatment. The performance of PSI for synthetic Cryptosporidium oocyst (S-Crypto) was higher than polyaluminum chloride (PAC) and ferric chloride (FC). The higher removal performance in Run-PSI (coagulation with PSI) was caused by both the biggest reduction of small particles and the higher sedimentation rate of residual particles and flocs produced. Before the sedimentation stage, PSI could reduce the numbers of the small particles and flocs up to half and one-third of that in Run-PAC and Run-EFC, respectively. It becomes 10 and 7 times lower at maximum after 29 min sedimentation in the comparison with PSI and FC, respectively. The predominant mechanism for S-Crypto coagulation was charge neutralization and interparticle bridging in Run-PSI. The biggest reduction of small particles and the higher sedimentation rate of residual were caused by the polymerized silica, not by iron in PSI.