There are several methodology based on voice analysis to evaluate nasal airway. Here we introduce a new quantitative index based on voice spectrum analysis to evaluate nasal obstruction. Ten subjects of nasal blockage were instructed to produced the sustained consonant-vowel syllable /m partial partial differential/ at comfortable levels of speech for at least 5 seconds. After nasal decongestant treatment, the second voice sample was collected. Sound spectrum was obtained by the algorithm of fast Fourier transform and the fundamental frequency (F0) was calculated by the method of autocorrelation. Voice low tone to high tone ratio (VLHR) was defined as the division of low frequency power (LFP) into high frequency power (HFP) of the sound power spectrum and was finally expressed in decibels. The cut-off frequency was the product of F0 and square root of (4 x 5). The VLHR after nasal decongestant treatment increased significantly as compared with that before treatment (P < 0.01). VLHR is a new index derived from sound spectral analysis and that may detect the changes in frequency characteristics of voice during treatment for nasal obstruction. The index is quantitative, non-invasive, and potentially useful for basic researches and clinical applications.