Abstract: The effect of electrostatic field on the photodegradation of Methyl Orange (MO) with P25 TiO₂ catalyst was investigated.The results showed that the effect of the electrostatic field is very different from the traditional electrically assisted photocatalysis (EAP).With the increase of the electrostatic field,the apparent photodegradation activity declined.The fastest degradation was obtained atc (TiO₂)=0.5 g/L without electrostatic field.However,the degradation rate of the same system was lower under higher electrostatic fields.The apparent rate constant of the degradation reaction under 20 kV electrostatic field was only 56% of that in the same system without electrostatic field.Furthermore,under the 20 kV field,the photodegradation was independent of the dosage of TiO₂.By comparing the particle size distribution and the sedimentation rate within and without electrostatic field,it was conduded that the effects of electrostatic field,such as the electrophoresis,the polarization and the destruction of surface charge layer,made the particles aggregate and deposit.The coagulation decreased the amount of the catalyst dispersed in the solutions,and finally,decreased the apparent rate constant of the degradation reaction.At the end,it was proposed that the coagulation effect could be used to recycle the TiO₂ nanoparticles from the photodegraded wastewater.