Abstract: Objective To evaluate the virulence of Klebsiella pneumoniae isolated from infant formula milk powder and to observe the pathological changes caused by the bacteria.Methods Median lethal dose(LD₅₀) was calculated by injecting different concentrated suspensions of a Klebsiella pneumoniae standard strain and five strains isolated from infant formula milk powder samples into mice abdominal cavity.The clinical symptoms of the injected mice were observed and the pathological changes of died mice were observed with gross anatomy and pathologic section preparation.Results The LD₅₀ for the five isolated strains and the standard strain were 3.031, 2.638, 1.516, 2.638, 1.516 and 1.516 McFarland(McF)/kg BW, respectively.Many organs and tissues of the injected mice presented obvious pathological changes, and the damages of lung and liver were more serious.Conclusion Klebsiella pneumoniae isolated from infant formula milk powder has strong virulence, and their contamination may result in potential hazards to infants.