This study was designed to explore the composite scores under the multivariate distribution of latent proficiencies using a procedure based on IRT for empirical data. The purpose was to employ the IRT models under the multivariate ability distribution in order to investigate the composite scores with the consideration that correlations existed among the examinees’ proficiencies.  This research followed Kolen, Wang, and Lee’s (2012) method for the development of the IRT-based procedure.  The five tests of the Basic Competence Test were used, with a random sample of 5,000 examinees obtained in 2008.  The analyses included the descriptive statistics and frequency distributions of the composite scores, overall SEM and reliability values, as well as the CSEMs.  The findings indicated that there existed strong relationships between pairs of the proficiencies of the five tests and the results of Kolen et al.’s modeling were satisfied.  For large-scale testing programs that rely on two or more tests to make high-stakes decisions, the issue of combining individual scores into a single total score is critical.  Through the employment of Kolen et al.’s model for deriving the composite scores under the multivariate distribution, and along with this research utilizing the real data, the results of this study have revealed more of the psychometric features of the composite scores and have also helped lay a more solid foundation for many studies to embark on the estimation of examinees’ ability levels in the multivariate proficiency setting via IRT.