Abstract: Objective Based on conventional pulsed-field gel electrophoresis(PFGE),we established a modified assay to screen plasmids of bacteria and applied it in the screening of Legionella plasmids. Methods L. dumoffii strains with and without plasmid were used for the optimization of the treatment parameters, cell suspension density, content of proteinase K and its action time, wash times were adjusted, then the best conditions were chosen. Results Compared with conventional PFGE assay, the procedure of our assay was without restriction digestion. Intact plasmid DNA and chromosome DNA could therefore be kept in the agarose plug without being denatured and breakage. Plasmid DNA and chromosome DNA can be separated by the electrophoresis. Conclusion Modified PFGE processes is more accurate and rapid than conventional assay and could be applied in the screening of plasmids of bacteria.