The main purpose of this research project are threefolds. First, to examine the pattern of changes in the Thai tax structure during the process of the country's economic development which started from the first to the fourth economic development plan (A.D. 1961-1971), and to see whether the existing structure promoted economic development by considering its effects on economic growth and income distribution. Second, to assess Thailand's performance in mobilizing domestic resources for development purpose by using an index of tax effort. Third, to evaluate the revenue performance and the stability performance of the Thai tax system by utilizing tax buoyancy and tax elasticity methodology to measure the responsiveness of revenue structure to Gross National Product. It was found out that changes in the Thai tax structure during the process of her economic development plans were such that income tax gained a higher proportion in the revenue structure, offsetting by a decline in the proportion of other taxes. The contribution of indirect taxes to total revenue was relatively stable, but there was a shift in the emphasis from taxes on foreign trade to taxes on domestic trade. The Thai tax structure as a whole promoted economic growth, but at the expense of a wider gap in income distribution. The major taxes which produced this effect were business tax and import tax. Thailand's tax effort as measured by the ratio of total tax revenue to Gross National Product or, the actual tax ration, was only 13 per cent which was relatively low as compared to those of other developing nations. In addition, when factors affecting the taxable capacity of the country such as income per capita, foreign trade ratio, and share of the agricultural sector were considered, the finding is that only the share of the agricultural sector significantly and negatively determined the tax ratio. A decline in the share of the agricultural sector by 1 per cent resulted in an increase in the tax ratio by 0.30 percent. Finally, the revenue performance and the stability performance of the Thai tax system were not quite satisfied as both the tax buoyancy and the tax elasticity coefficients were only slightly more than one. Although direct were relatively income-elastic, their small contributions to total revenue rendered their roles in financing and stabilizing the Thai economy quite limited.