Aims: The aim of this work was to clarify the effects of electromagnetic wave irradiation (EMWI) on oral bacterial pathogens. Methods and Results: A Gram-negative (Porphyromonas gingivalis) or Gram-positive (Streptococcus mutans, S.intermedius, Enterococcus faecalis) bacterial suspension was irradiated by EMW apparatus (5001000 kHz, 515 times, 1 s time-1). Quantification of survival bacteria by CFU counting revealed that EMWI exhibited marked bactericidal activity against all tested bacteria and bactericidal activity at 500 kHz increased in an irradiation number-dependent manner. After EMWI at 500 kHz, scanning electron microscopic observations showed that the chain of S.mutans cells was shortened after 5 irradiations and the outlines of bacterial cells (S.mutans and P.gingivalis) were unclear after 510 irradiations. EMWI inhibited the inductive effect of S.mutans on pro-inflammatory cytokine production in human monocytes and this inhibitory effect was comparable with that of heat-killed bacteria. Furthermore, using an enzyme activity assay, EMWI partially inactivated the activities of gingipains from P.gingivalis. Conclusions: These findings demonstrated that EMWI has inactivation and bactericidal activities against single microbial species among four kinds of oral pathogens. Significance and Impact of the Study: Electromagnetic wave irradiation may be applicable for medical disinfection and sterilization, such as refractory periapical periodontitis.