The degradation processes of natural stone building are controlled by natural and anthropogenic factors. The environmental factors as humidity, temperature variations, wind, and air pollutants together with microbiological activity lead in time to stone decay. Mineralogic, petrographic and geomycological investigations were performed on the natural stone from Ursuline Monastery entrance portal, located in Oradea downtown, Romania. Optical transmitted light microscopy and X-ray diffraction were used in order to investigate the mineralogy and petrography of the samples collected from the investigated stone portal. Based on mineralogical and petrographic results, interpretations in connection with the effect of some atmospheric factors as humidity, air temperature, solar radiation, wind, air pollutants respectively, as well as with identified fungi (Penicillium sp., Alternaria sp., Aspergillus sp., Cladosporium sp., Cryptococcus laurentii), were made in order to evaluate de stone decay.