Objective: The study was conducted to examine the psychometric properties of the Insomnia Severity Index-Chinese version (ISI-C).Method: Study 1 recruited 147 insomnia patients from a general hospital and 182 normal sleepers from local communities. A package of questionnaires including the ISI-C and the Chinese version of the Pittsburgh Sleep Quality Index (CPSQI) were administered to all subjects. Study 2 included 71 patients with insomnia complaints who went through a 6-session program of cognitive behavioral therapy for insomnia (CBT-I). The ISI-C and sleep diaries were given to the participants before and after the CBT-I program.Results: The results showed very good internal consistency of the ISI-C, with a Chronbach's α coefficient of 0.94. Corrected item-total items correlations were all significant. In terms of validity, ISI-C showed very good concurrent validity with the CPSQI. The total scores of these two scales correlated significantly (r = .88). The correlations between most of the items of the two scales also reached moderate to high correlations. The correlations between ISI-C items and corresponding indices derived from sleep diaries were also significant, although most of them were in the low to moderate correlation range. Furthermore, patients' scores on the ISI-C decreased significantly after receiving CBT-I program, reflecting the beneficial effects of treatment. Regarding construct validity, factor analysis revealed a single factor that can explain 73.94% of the variance. ROC analysis showed a best cut-off point of 9, with very good sensitivity (91.8%) and specificity (91.2%).Conclusion: The ISI-C has good internal consistency and validity. It can be a sensitive and specific tool for screening insomnia and a good outcome measure to evaluate the treatment effects of insomnia treatments.