OBJECTIVE To study the germicidal efficacy and germicidal mechanism of compound ortho-phthalaldehyde(OPA) against Bacillus subtilis var.niger spores.METHODS Suspension quantitative germicidal test was used to observe the germicidal efficacy of compound OPA against B.subtilis var.niger spores.The germicidal mechanism of compound OPA was studied in transudation of protein and DNA and ultra-microstructure.RESULTS The results showed that at(20?1)℃,the logarithm mean reduction was greater than 5.00,after B.subtilis var.niger spores had been exposed to compound OPA for 30min.Some protein leaked from B.subtilis var.niger spores treated with compound OPA for some time,but there wasn′t DNA.The ultrastructure showed that the shape was amorphic,the electronic density of the coat was unequal,the core leaned to one side or appeared the transmission area.CONCLUSIONS Compound OPA can effectively kill B.subtilis var.niger spores.Its mechanisms are that in accordance with benzolkonium bromide(Bromogeramine) the compound OPA can change the osmotic pressure of cell and increase cell permeability,then it can easily penetrate to their target sites within the cell.