Aramid fiber (AF) was treated by NaOH solution of various concentrations with supersonic vibration at room temperature. The density of NaOH solution was the main parameter. Elongation at break and break strength were tested. The chemical composition of AF was characterized by elemental analysis. The surface of AF was examined by scanning electronic microscopy (SEM). It was shown that elongation at break of treated AF was not decreased, and break strength was a little increased, first increases and then decreases as the concentration of NaOH solution raising. It was also found that the surface of AF after modified was a little rougher. Low concentration of NaOH solution was fit to modify AF.