AIM: To explore the relationship between caspase activation and the evasion of Legionella from macrophage elimination through a Legionella-infected macrophage model. METHODS: After infected by Legionella, the activity of caspase 3 in macrophages was analyzed by confocal microscopy as well as fluorescence reader. Growth and replication of Legionella in macrophage was assayed. Replication of Legionella was analyzed again to see the effect of caspase 3 inhibition on the growth of Legionella after use of caspase 3 inhibitor. RESULTS: Both confocal microscopy and caspase 3 fluorescent substrate analysis showed that Legionella virulent strain had powerful capability of activating caspase 3 while the mutant non-virulent strain did not have this capability. The virulent strain highly replicated in macrophages and the replication was significantly inhibited by caspase 3 inhibitor. CONCLUSION: Our results indicate that the intracellular caspase 3 is activated shortly after infection by Legionella virulent strain. The evasion of Legionella from the elimination of macrophages may be mediated by caspase 3 activation to a great degree.