Quantum dots (QDs) are semiconductor nanocrystals whose unique fluorescence properties make them desirable biological imaging probes. However, reliable and efficient cellular delivery of QDs remains technically challenging. To address this problem, we developed a cell penetrating peptide (CPP) based approach that delivers QDs into mammalian cells with high reproducibility and efficiency and minimal cytotoxicity. To understand the delivery mechanism, we analyzed related cell uptake pathways. We followed internalization and endosomal release of CPP conjugated QDs (CPP-QDs) and found that although endocytosis (micropinocytosis) was the predominant pathway, some CPP-QDs were internalized through direct permeation of the plasma membrane. Internalized QDs could be released from endosomes to the cytoplasm if conjugated with an endosomolytic peptide (HA2), but most of released particles either were re-captured by lysosomes or aggregated in the cytoplasm. Together, our results provide insights into mechanisms of CPP mediated cellular delivery of quantum dots for intracellular imaging as well as therapeutic applications.