Purpose: Air as a source of microorganisms has an important role in the biological contamination and serious bio-deterioration of archival materials, so regular microbial monitoring of the air of repositories along with protective conservation policies for identifying and correcting critical conditions, is inevitable. The aim of this study was to evaluate the biological air contamination levels in the archival repositories of the National Library and Archives of Iran, by active and passive sampling methods, and to assess the relationship between the results of the two methods.Method/Research Design: Air sampling of repositories was carried out in two ways: Active (using Air sampler model, SAS), and Passive (Settle plates) methods, according ISO 14698-1: 2003, standard. After determining of TVC (Total Viable Count) and identifying different groups of microorganisms by microbiological methods, the correlation between the results of the two methods and frequency of different groups were evaluated.Findings: Among the isolates of fungal genera the highest frequency is reached by Penicillium, Cladasporium and Aspergillus in both passive and active sampling methods. Prevalence of contaminant bacteria of the air of repositories, in both sampling methods, indicates dominance of sporulated gram-positive rod bacteria such as Bacillus and Streptomyces. Statistical analysis using SPSS confirmed the correlation between the two methods of air quality assessment. Frequency of different groups of micro organisms introduces active air sampling as a better method for isolation of fungi genera.Conclusion: The results of both methods are reliable for periodical monitoring of air microbial contamination.