For evaluating the cigarette smoke quality of nonblended and commercial cigarettes, four modified smoke quality indexes were deduced from considering the reactivity of the thiol group in cystine (SH value) with the cigarette smoke constituents, and from the maximum limited contents of tar. nicotine, and carbon monoxide for the less harmful cigarettes. These results were discussed with respect to correlation coefficients between the indexes and tar/nicotine ratio which had been known reliable for smoke quality evaluation. Among the three indexes, the SH/[(N/O.6) +($frac{C+T}{8}$)] index was found to be the most reliable and applicable.