Abstract: Effect of tricyclazole on secondary infection by rice blast fungus, magnaporthe grisea Barr., and mechanism of this inhibitory effect were studied by imitating the complete process of secondary infection in laboratory. The results showed that the curative efficiency of tricyclazole against rice blast was weak. The efficiency of 10 μg/mL tricyclazole spraying treatment 48 h after inoculation was only 28.58 %. But the secondary infection to nearby untreated healthy plants caused by disease plants treated with 10 μg/mL tricyclazole as above was 63.28% less serious than that caused by the untreated disease plants. Further, inhibiting sporulation,hindering liberation and reducing pathogenicity of spores from the treated disease plants were the prominent mechanisms of this effect.