Conventional electrostatic precipitators (ESPs) used for exhaust gas treatment equipment and air cleaners have generally been said to have a penetration window in a submicrometer particle size range. Further, recent works have shown that the collection efficiency of particles with a diameter less than about 50 nm decreases due to poor charging by the corona. In this paper, we describe an attempt to use in situ soft X-ray irradiation to enhance the ultrafine particles in the ESP. A laboratory-scale ESP was configured to irradiate particles with soft X-rays. The collection efficiency of the particles was then measured with and without irradiation to examine the enhancement. The results showed a noticeable improvement in the efficiency of ultrafine particles (d&lt
sub&gt
p&lt
/sub&gt
&lt
60 nm) with irradiation. In particular, X-ray irradiation combined with corona discharge was much more effective than irradiation without it. Further, a higher efficiency was obtained with a positive corona than with a negative one.