Present invention relates to an oxidizing composition for keratin fibers especially for human hair with improved lightening effect and therefore, improved colouring and bleaching effects are easily achieved of the compositions applied after mixing with the said composition. Present inventors have surprisingly found out that a composition comprising at least one oxidizing agent and additionally at least one alkalizing agent, at least one carbonate source and at least one urea derivative delivers an improved and homogeneous lightening performance and leaves hair in an improved cosmetically acceptable conditioned status.