A multiplex PCR assay system was developed for the detection of Clavibacter michiganensis subsp. Michiganensis (Cmm), which combined two tests in one reaction mixture. Cmm-specific primers PSA-4/PSA-R and Solanum lycopersicum-specific primers NS-7-F/NS-8-R (inteal PCR control primer) were combined in one PCR reaction mixture with Cmm and plant DNA as template. The primer sets could amplify the target product successfully. Different combinations and concentrations of primers and annealing temperatures were tested, respec tively. The detection level of the optimized multiplex PCR assay was up to 5×l02cfu-mL-1. To verify the applicability of this system, it was employed to detect Cmm in tomato seeds and plantlet samples. Seeds mixed with Cmm and diseased plantlets were detected successfully. The multiplex PCR system will avoid false-negative results and provide a reliable method for the detection of Cmm.