Abstract: To investigate the impacts of indole and 3-methylindole on the aroma of mainstream cigarette smoke, a quantitative GC/MS method was developed for analyzing the indole and 3-methylindole levels in total particulate matter (TPM) of mainstream cigarette smoke. The contents of the two analytes in TPMs of mainstream smoke of 14 cigarette brands were determined. The olfactory thresholds of indole and 3-methylindole, as well as the concentrations at which their olfactory characteristics changed, were determined in ethanol and triacetin separately. Their odor activity values (OAVs) in TPMs were also calculated. The results showed that:1) The olfactory threshold of 3-methylindole was much lower than that of indole, irrespective of ethanol or triacetin. However, the difference between their contents in TPMs was less than one order of magnitude, therefore the OAVs of 3-methylindole were much higher than those of indole. 2) The olfactory thresholds of indole and 3-methylindole in ethanol were much lower than those in triacetin, and their olfactory characteristics changed from floral to fecal with increasing concentrations. 3) Indole and 3-methylindole had important effects on the aroma of cigarette smoke, especially 3-methylindole. 4) The contents of indole and 3-methylindole in all investigated TPMs were much higher than the concentrations at which their olfactory characteristics changed, but the fecal odor of indole and 3-methylindole in cigarette smoke could not be sensitively felt due to the masking effect of smoky aroma in cigarette smoke. Their accumulation in buccal cavity might be the reason for the unique smell in the breath of long-term cigarette consumers.