The characteristics of inactivation and photoreactivation of Escherichia coli by a high-power pulsed ultraviolet (PUV) irradiation and the influence of the humic acids on disinfection performance were investigated. The pulsed power source was operated at 2,400 V, with 200 J of energy being stored in a 100-μF capacitor. This energy dissipated in the xenon-filled flashlamp within 150 μs, generating a megawatt-level peak power per pulse. The light source was operated at 12 pulses per second (12 Hz). More than a 6-log reduction was achieved via 5 sec of irradiation at a distance of 20 cm, which corresponds approximately to a UV dose of 23 mJ/cm2. The inactivation efficiency decreased with increasing concentrations of humic acids (HA). At the distance of 20 cm, 10 ppm HA reduced the inactivation efficiency to 50%. UV absorption by HA and, in part, the growth-promoting effect of humic acids were implicated in reductions of inactivation performance in the presence of HA. After a disinfection procedure with PUV, the possibility of photoreactivation was greatly reduced and an additional reduction of approximately 1-log was also achieved during 8 h under visible light after inactivation with a UV dosage of 9.0 mJ/cm2 (3 sec at 30 cm). The extent of additional inactivation under visible light was reduced with increasing concentrations of humic acids in water.