The principle of tax equity is the most important principle for taxation system design and revision. However, it has tax preference based on consideration of tax collection and social policy. According to Article 14, Category 6 of Individual Income Tax- Self-undertaking in Farming, Fishing, Animal Husbandry, Forestry and Mining, it regulates that the total income is the whole year’s income after deduction of necessary expenses. Furthermore, the necessary expenses are 100% of revenue for whole year based on legal interpretation which is promulgated by Ministry of Finance. Then it has tax preference actually. Nevertheless, it is against principle of tax equity. We analyze formation and development of policy from the perspective of historical literature in this study. We find that it is not exemption on the Self-undertaking in Farming, Fishing, Animal Husbandry, Forestry and Mining initially. Furthermore, it should have space to revise based on consideration of tax collection, social policy and shift of social economic structure. Hence, we propose that government should review that costs and expenses again in order to conform to tax equity.