Revenue from personal income tax is one of major sources of government tax revenue in many countries. It is collected at source. The tax incidence is theoretically borne by tax payers. Any attempt to improve the personal income tax structure is expected to result in an increase in revenue from direct tax. To this effect, the government will have ample leeway to introduce new measures to further development the economy. The main objectives of this thesis are twofold. First, the study analyses changes in the personal income tax structure in Thailand from 1981-1993 including their impacts on the Thai economy. Secondly, the study attempts to utilize econometric model to forecast the personal income tax revenue. The empirical study is based on secondary data from 1977-1993. The model comprises two modules. One is for income tax and the other is for macro-economic relationship. The highlight of the study can be elucidated as follows. It finds that the average tax paid per person increased from 3,579 Baht to 15,270 Baht from 1981 to 1993. In addition, the GDP elasticity of personal income tax from interest is 2.69%. Finally, the model predicts that the growth rate of personal income tax revenue will markedly decline. As a result, this can send an alarming signal to the Thai government especially the Ministry of Finance.