Abstract: To investigate possible reasons behind the deterioration of base casing for tobacco, tobacco microbial population, colony quantity and aroma components were analyzed. Using common separation medium, 28 bacteria and 1 fungus were isolated and identified from the normal and deteriorated base casings for cigarette brands "Purple Yun Yan" and "Soft Precious Yun Yan". The fungus belonged to Saccharomyces sp. and 28 bacteria belonged to 12 genera, including Bacillus sp., Microbacterium sp., Sphingomonas sp., Leclercia sp., Acinetobacter sp., Brevundimonas sp., Agrobacterium sp., Ralstonia sp., Acetobacterium sp., Acidovorax sp., Burkholderia sp., Enterobacter sp. The total microorganism populations in the two deteriorated base casings were 90 and 29 times of those in the normal casings for the same brands, respectively. The dynamic variations of total microorganism population were investigated, the results showed that base casing started to deteriorate from the 6th day at room temperature. The types and contents of aroma components in normal and deteriorated base casings were detected by two-dimensional gas chromatography-mass spectrometry (GC-MS) and the results indicated that the types and contents changed greatly after deterioration. In the deteriorated base casing for cigarette brand "Purple Yun Yan", the contents of β-damascone and δ-damascone changed most significantly, while menthol was not detectable. In the deteriorated base casing for cigarette brand "Soft Precious Yun Yan", the contents of octanoic acid, decyl acid, ethyl ester, β-damascone and δ-damascone changed most significantly, while octanoic acid ethyl ester, menthol, 3, 5, 5-trimethylcyclohexane-1, 2-dione, benzaldehyde, α-damascone were not detectable.