Abstract: Using an increasing dose pressure, a piperaquine-resistant line (PR line) in P. berghei K173 strain was developed. The starting dose was 7 mg/kg, about the total piperaquine ED₅₀ dose in the normal strain (parent line). The dose was increased by 3.5 mg/kg every three passages. Resistance detection test showed that the resistance level of PR line in passage 7 was 3.9 times that of the parent line, up to 75 times in passage 13, and>110 times in passage 20 in 5 months.The piperaquine-resistant line exhibited clear cross resistance to hydroxypiperaquine, mefloquine, qinghaozhi (artesunate), and qinghaosu (artemisinine). The resistance index (ED₅₀ in PR line/ED₅₀ in parent line) were 95.15, 45.27, 3.82, and 3.30 respectively. Its cross resistance, however, was slight to pyronaridine, chloroquine and primaquine, the resistance index being 1.55, 1.29, and 1.26, respectively. Towards pyrimethamine and nitroquine, the PR line displayed some hypersensitivity, the index being 0.82 and 0.55.With the augmentation of resistance, the course of infection of the PR line in mice was longer than that of parent line, indicating that the virulence dropped while resistance was raised. Serial drug-free passages showed that the resistance of PR line was unstable. Although the resistance level was not changed from passage 1 to 4, it dropped markedly from passage 5 onwards, and decreased from>110 to 8-fald after nassages 12.