Bisphenol-A dicyanate (BADCy) resin was toughened by incorporating polysulfone (PSF) and cyanated polysulfone (CN-PSF). CN-PSF was synthesized by the bromination of PSF and cyanation. The effects of PSF content and interfacial adhesion between the matrix and the domain were investigated on the fracture toughness and morphology of BADCy/PSF blends. PSF formed a matrix Phase when more than 20 parts per hundred of resin (phr) of PSF was incorporated. The particle size of BADCy decreased with increasing numbers of cyano groups in PSF. There was an optimum cyano group content for maximizing the fracture toughness of BADCy/CN-PSF blends with 30 phr of CN-PSF content. Toughening mechanisms were examined using scanning electron microscopy and transmission optical microscopy.