Dabrafenib Mesylate (1), a typical anti neo plastic agent drug, is used for the treatment of skin cancer. During the laboratory optimization, formation of an unknown impurity at RRT ~ 0.94 was observed in the final Dabrafenib. Origin of the impurity was possible when chloro pyrimidine intermediate (15) was converted to Dabrafenib (1).In that, the aromatic fluoro group gets converted to amine which leads to the impurity formation (Phenyl amino Dabrafenib impurity (2)). This impurity was prepared and characterized by using spectroscopic techniques. Herein the formation, synthesis and characterization of this impurity is described. Our study will be of immense help to others in obtaining chemically pure Dabrafenib Mesylate.