Cancer is one of the major health problems with an increasing incidence and mortality. Although imaging and effective treatment methods are critical issues to prevent cancer mortality, they are inadequate. Therefore, new approaches including combined treatment protocol and nanosized drug delivery theranostic systems are being investigated because of the deficiency of conventional cancer imaging and treatment methods in clinics. Liposomes attract attention in cancer imaging and treatment due to their advantages. In this study, an HPLC method was developed for simultaneous quantification of paclitaxel (PCX) and vinorelbine tartarate (VNB) in co-drug encapsulated theranostic nanosized liposome formulations as an effective cancer imaging and treatment agent. The method was validated according to ICH Analytical Method Validation guideline. According to the validation results, the method was found to be linear (r > 0.999), accurate (bias <= 1.8), precise (RSD <= 1.8%) and robust (p > 0.05). The developed method was successfully applied to the quantification of PCX and VBN in liposomes for encapsulation efficiency.