Human cytomegalovirus UL79 protein is recently reported to be required for transcription or efficient accumulation of late viral mRNAs during viral infection. An absolute nuclear distribution of UL79 proteins has been observed with immunofluorescence assay, both during the infection of Flag-tagged UL79 recombinant virus and in the HFFs expressing HA-tagged UL79, with or without virus infection. However, little is known about the nuclear import mechanism of UL79 protein. Here, by utilizing living cells fluorescent microscopy, a predominant nuclear localization of UL79 protein in living cells was detected. Furthermore, the nuclear import of UL79 protein was demonstrated to be dependent on the transportin-1-mediated pathway. Finally, a hydrophobic PY-nuclear localization signal (PY-NLS) was delineated between the amino acids 66-92 of UL79 protein. Collectively, we provide evidence that a PY-NLS, firstly described in viral proteins, is responsible for the nuclear accumulation of UL79 protein.