[Objective] To evaluate the infectivity of Cryptosporidium parvum oocysts in NMRI suckling mouse.[Methods] Four-day- old SPF NMRI suckling mice were inoculated with different amounts of oocysts by oral gavage.On clay 7 after inoculation, suckling mice were sacrificed, and a suspension was prepared by homogenizing the intestinal tract from pylorus to anus. A mouse was considered infected when oocysts were found in smears of the intestinal content suspension stained with carbo lfuchsin solution. The infectivity of oocysts was evaluated as measured by the percentage of infected mice in each group. [Results] Mice receiving 1 500 or 2 000 oocysts were all infected. The percentages of infected mice were 88, 74, 51 and 28 respectively after ingestion of 1 000, 500, 250 and 100 oocysts. The percentage of infected mice was 9.5 % after ingesting as few as 50 oocysts. [Conclusion] This model is convenient for evaluation of the infectivity of C. parvum oocysts.