The preparation and   evaluation of a theranostic conjugate composed of trastuzumab, paclitaxel (PTX), and deferoxamine (DFO)-chelated Zr have been reported. These comounds have potential applications against HER2 receptor positive breast cancers. We conjugated DFO and PTX to trastuzumab by exploiting simple conjugation chemistry. The conjugate (DFO-trastuzumab-PTX) showed excellent radiolabeling efficiency with Zr and the labeled conjugate had high in vitro stability in human serum. Furthermore, DFO-trastuzumab-PTX displayed comparable cytotoxicity with PTX and Zr-DFO-trastuzumab-PTX exhibited HER2 receptor-mediated binding on HER2-positive MDA-MB-231 breast cancer cells. The results of our in vitro study indicate high potential of Zr-DFO-trastuzumab-PTX to be utilized in the theranostic application against HER2-postive breast cancers.