The aim of this study is to test the psychometric properties and validity of the World Health Organization Assessment Schedule II Chinese Traditional Version (WHODAS II CT) in Traditional Chinese-speaking persons with disabilities and chronic illnesses.The WHODAS II CT has been administrated to a sample of 1020 persons with disabilities and chronic illnesses. The construct validity, internal consistency, concurrent validity and convergent validity were evaluated.WHODAS II CT showed a satisfactory model fit for the second-order confirmatory factor analysis model (χ(2)/df = 3.05, root means square error of approximation = 0.053, comparative fit index = 0.912, standardized root mean square residual = 0.076), high internal consistency (Cronbach's α = 0.98), high correlation with all domains of Dartmouth Primary Care Cooperative Research Network/World Organization of National Colleges, Academies, and Academic Associations of General Practices/Family Physicians (COOP/WONCA) charts (partial correlation coefficient ranged from 0.26 to 0.74) and significance between persons with and without co-morbidity (all regression coefficients0).WHODAS II CT is a reliable and valid instrument to measure the disability in persons with disabilities and chronic illnesses among Traditional Chinese-speaking population. A further study is required to validate the short version of WHODAS II in order to enhance its applicability in usual and clinical practices. Implications for Rehabilitation This is the first study to evaluate the reliability and validity of WHODAS II in persons with disability and chronic illnesses among Traditional Chinese-speaking population. The WHODAS II CT is a valid instrument in Chinese adults with disabilities and chronic illnesses. The WHODAS II CT is recommended to be used in population-based survey to investigate the health needs of persons with disabilities and chronic illnesses as well as in the rehabilitation programs as an outcome measure.