Human Blastocystis hominis were isolated from diarrhea patients' feces and cultured in vitro. Then the cultures were inoculated intraperitoneally to laboratory mice. The B. hominis in living mice were collected and inoculated again to healthy mice. The B. hominis showed dose-dependent pathogenicity in the primary inoculation. No pathogenicity was observed in the secondary inoculation. The protozoan existed in the living mice abdomen cavity for more than 6 months and the cyst was the only form. These results showed that encystation enable the parasite to avoid the immune attack in competent host and simultaneously decrease the pathogenicity to host. Intraperitoneal inoculation to laboratory mice is a good method to maintain and propagate B. hominis. This is also a good model to study the interaction of B. hominis and immune system.