Thailand is in the demographic transition phase. The shape of population pyramid is shifting from stationary to contracting pattern. Age-sex distribution may vary by province. This study explores and describes the population distribution pattern of 76 provinces in Thailand using data from 2000 Thai population census. Factor analysis, a multivariate statistical method, was used to cluster provinces, based on pattern of age-sex distribution of the population. The study found three distinct patterns of population distribution in Thailand. Twenty-seven southern and northeastern region provinces, mainly bordering Myanmar, Cambodia or Malaysia, share the classical pattern of population distribution. The majority of central region provinces, and also Phuket from the south share a similar population distribution pattern which peaks at the young age group. So too, most of the northern region provinces share another pattern that dips at the young age group. In conclusion, this study found that population distribution is not symmetrical across Thailand. The factor model approximated well this variation and clustered the provinces in three patterns. The method applied in this study is straightforward and can be used in future demographic studies