In this study, normal and dichlorodicyanuric acid (DCCA)-treated wool slivers and yarns were immersed in a formic acid solution at elevated temperatures as a pre-treatment in order to accelerate the scale removal and fiber splitting of wool by ultrasonication and the effect of pre-treatment on the morphology and fiber splitting of wools was investigated. As the formic acid concentration, the treatment temperature, and the treatment time increased, the scale removal on wool fiber increased, however, the occurrence of felting and yellowing regarding wool fiber also increased indicating that the treatment with 50% formic acid at $100^{circ}C$ for 3 hours was the optimum pre-treatment condition for wool fiber. The SEM observation revealed that the formic acid treatment on wool at $100^{circ}C$ followed by ultrasonication was the effective tool in order to produce split fibers for both normal and DCCA-treated wool. The comparative study of sliver and yarn revealed that ultrasonication could lead to the fiber splitting of wool sliver whereas it could not result in that of wool yarn. On the other hand, formic acid treatment at $100^{circ}C$ resulted in fiber splitting for both wool sliver and yarn.