Abstract: The assays of therapeutic efficacy of Toltrazuril against I. suis in 7-day-old piglets were conducted. Its efficacy was evaluated by reduction of diarrhea, decrease of oocysts production and mortality, improving body weight gain and so on. The results revealed that I. suis was negative in the faeces of piglets treated with Toltrazuril (groupⅠand Ⅱ) 3 days and 4 days after treated with Toltrazuril, respectively. While I. suis still was positive in that of piglets untreated with drugs beyond one week. Diarrhea disappeared in groupⅠand Ⅱ 5 days and 6 days after treated with Toltrazuril, respectively. It was significant that difference of body weight gain between piglets treated with Toltrazuril in groupⅠand Ⅱ(65.1% and 61.9%,respectilvely) and piglets untreated in group Ⅲ. In addition, Toltrazuril can significantly decrease mortality and remarkably improve appetite of piglets. It concluded that Toltrazuril was effective evidently against I. suis.