In this study, we examined the micropore-size control on activated carbons by ultrasonic irradiation with various conditions. Pitch-based activated carbon fiber (ACF) was used for the sample having relatively uniform micropores whose micropore size was determined as 1.15 nm from the measurement of nitrogen adsorption isotherm at 77 K and high-resolution αs-plot analysis. Also, the ultrasonic irradiations to ACF were carried out in the water bath with various irradiation times and at three irradiation frequencies (40 kHz, 200 kHz, and 1 MHz). The results indicate that we can decrease the pore volume and average pore widths of ACFs with longer irradiation time. Also, we examined the dependence of ultrasonic frequencies on micropore-size control. The average pore width of ACF irradiated by 40 kHz was decreased over 10% from the original one. This result suggests that the nanospaces of ACF are collapsed as a result of sonication. Besides, larger pores, are more easily collapsed selectively.