Doxorubicin loaded micelles were prepared by film-hydration method using stearyl sulfadiazine (SA-SD) which is pH sensitive, methoxy (polyethylene glycol)-2000-1, 2-dioleoyl-sn-glycero-3-phosphoetha- nolamine (mPEG-DOPE) and transactivator of transcription (TAT) peptide conjugated PEG-DOPE.  Mean diameter of the pH-sensitive micelles was about 20 nm with a (99.1 ± 2.1) % drug entrapment efficiency at pH 7.4.  Flow cytometry studies revealed that the simple TAT micelles was taken up rapidly at the same level at pH 6.8 and pH 7.4.  However, the pH-sensitive micelles entered the tumor cell less at pH 7.4 and significantly increase at pH 6.8.  After 1 h incubation at pH 6.8, the amount of the pH-sensitive micelles taken up by cancer cell 4T1 was almost similar to simple TAT micelles.  The confocal microscopy indicated that the pH-sensitive micelles entered the 4T1 cells at pH 6.8 more than at pH 7.4.  It was indicated that the pH-sensitive micelles could shield TAT peptide at normal pH 7.4 and deshield it at pH 6.8.  Hence, TAT peptides lead the drug-loaded micelles into the tumor cells and killed them selectively.  The pH-sensitive micelle may provide a novel strategy for design of cancer targeting drug delivery system.