To obtain information on the indoor air quality of un-disinfected libraries used for paper records preservation, temperature, humidity, and bacterial concentration and species were investigated at five un-disinfected libraries from May to September in 2019 in the National Archives, Seoul (Nara Repository), Korea. Temperature and humidity of all the five un-disinfected libraries were well maintained at 18-22oC and 40-55% in compliance with the National Archives Paper Records Preservation Environment Standard. Bacterial concentration ranged from 2 CFU/m3 up to 280 CFU/m3 which were lower than the Korean indoor air quality standard value. A total of 68 bacterial species belonging to 33 genera were identified from indoor air of the five un-disinfected libraries. Among the 33 genera, Bacillus, Micrococcus, and Staphylococcus were the major genera. Only Micrococcus yunnanensis, Roseomonas mucosa, and Moraxella osloensis were commonly found among the five un-disinfected libraries. Bacterial species producing colonies with color on TSA media were present. Among the 68 species, Bacillus circulans and B. megaterium known to produce cellulases were found. There were also 17 species which have harmful effect on human health. The results of this study indicate that continuous monitoring of air borne bacteria is necessary in the un-disinfected libraries used for paper records preservation.