Not only developed countries have been concerned about tax revenue losses due to e-commerce; developing countries have concerns about this form of commerce also. In a case study of Thailand, the result shows that tax revenue losses from e-commerce would be approximately 0.3% of total tax revenues if the sales tax is exempted from all e-commerce transactions. Services, banking and insurance sectors are expected to generate the largest tax losses. However, preventing tax losses is not a sufficient justification for the imposition of a new tax into the Thai tax system; other criteria such as efficiency, equity, and administrative costs should be considered. This study elaborates on these criteria and argues with current technological know-how, imposition of a new “e-tax” into the current Thai tax system could increase horizontal and vertical equity, but it would necessitate large administrative costs which could render the tax impractical.