Abstract: Objective To find the change of virulent gene expression and to analyze the relevance between the virulent change and the gene expression. Methods Grouped guinea pigs were inoculated with 1 mL Leptospira cultured in vivo, Leptospira cultured in vitro and the Leptospira culture medium through abdominal subcutaneous respectively. The survival rate, body mass and temperature change of guinea pigs in different groups were measured within 15 d after the inoculation, then the survived guinea pigs were scarified, and the organ coefficient was also measured to know the virulence of Leptospira cultured in different environment. The amplified gene segments from Leptospira were used as probes and wrote the microarray. The total RNA was extracted from Leptospirastandard strain cultured in culture medium and guinea pigs. After reverse transcription to cDNA, they were labeled with Cy3 and Cy5 respectively. Labeled cDNA was mixed and hybridized with the microarray. The hybridized mircroarray was scanned and analysed. Results The survival rate of inoculated guinea pig was different from group to group (in vivo group: 0%; n vitro group: 88.9%; culture medium group: 100%). The guinea pigs in vivo group had a higher temperature (P <0.05), lighter body mass (P <0.05), larger organ coefficient (P <0.05) and a more serious hemorrhage in lung. The genes from Leptospira: LA1027, LA1029, LA4004, LA3050, LA3540, LA0327, LA0378, LA1650, LA3937, LA2089, LA2144, LA3576, LA0011 and gene of Loa22 were up regulation after continuously cultured in guinea pigs. Conclusion The pathogenic ability of Leptospira cultured in different environment is different and the gene expression of Leptospirais different between in vivo and in vitroas well. The understanding of the meaning of this change might help to know the pathogenecity of Leptospira.