Abstract: Extracellular acidity has been associated with many pathological states, such as cancer, ischemic stroke, neurotrauma and infection, which makes it an effective target for therapy and diagnosis of such diseases. As a polypeptide vector, pH low insertion peptides (pHLIPs) are endowed with high sensitivity to extracellular acidic environment, which can insert the membrane and deliver payload to pathological cells in a pH dependent manner. Here, theranostic applications of pHLIP in disease, are reviewed in two aspects:pHLIP-mediated single-molecule transporter and nano-sized carrier.