The International Union for Conservation of Nature and the Korean Red Data Book both categorize the dragon swallowtail Sericinus montela Grey, 1852 (Lepidoptera: Papilionidae) as a vulnerable insect. In South Korea, the main reason for this categorization is habitat destruction and deterioration, along with limited distribution mainly to the eastern region of South Korea, particularly to Gangwon-do and Gyeongsangbuk-do. Considering much of the information is older than ten years the renewed survey to estimate distributional stability is required, particularly under global warming. In addition, the population genetic diversity and isolation of S. montela, which is important information when seeking to evaluate its vulnerability, is not yet known. In this study, we visited 38 habitable sites, consisted of 15 sites with the previous record for the sightings of the species and 23 sites without such record. Moreover, nuclear ribosomal internal transcribed spacer region was sequenced for 56 individuals collected from seven sites. We observed S. montela at 20 sites, consisted of 11 recorded and 9 new sites in seven provinces, excluding Jeollanam-do and Jeju-do. Finding of the species in 9 new sites and location of the 4 in western region, which has a rare previous record are promising. However, failure to observe in 4 recorded sites and an extremely low observed number in new sites (5.3 vs. 17.6 individuals per site on average) is not optimistic. Population genetic data partially corroborate with the field observation data, showing that the populations in western region have lower genetic diversity and intermittent genetic isolation compared to those in eastern region. Taken these together, we suggest that S. montela should be remained as a vulnerable insect. However, more extensive field monitoring and co-dominant molecular data are required to verify this conclusion.