This study investigates the sources of health improvement in Thailand during the greatest period of its advancement. In order to evaluate these sources, various health indicators and socioeconomic variables are used to analyze their causalities at multiple levels ; national, regional, and provincial. The multivariate regression results confirm the existing theoretical concept of health determinants. These findings reveal that education, living conditions, and health resources significantly have a positive impact on health, while the net effect of income on health is unclear as its effect differs between health indicators and between regional levels. The results also disclose that the local-specific factor is another important source of health improvement and health status disparity. In addition, the dominant effect of health resources on health improvement is found in a long time-series national model. These results contradict the previous findings derived from cross-sectional data. This difference leads to the argument that in long-run health resources definitely play important roles in improving health status of a population, particularly in developing countries (as Thailand) where health resources typically seem inadequate.