政府為達成其經濟政策，採行各項租稅優惠措施。長期實施租稅獎勵的結果，稅基嚴重遭受侵蝕，租稅的公平性亦遭到質疑。為改善國家財政困境及租稅不公平現象，政府對於特定免稅所得進行稅制改革，訂定「所得基本稅額條例」以實施最低稅負制，自2006年起對特定免稅所得課徵10％之最低稅負。本文探討實施最低稅負制對公司隱稅之影響，實證結果顯示最低稅負制實施前，公司之證券交易免稅所得與股票稅前報酬率呈現負向關係，此符合隱稅理論，即租稅優惠愈大，隱稅成本愈高，稅前報酬率愈低。而在最低稅負制實施後，公司證券交易免稅所得與股票稅前報酬率呈現負向關係之幅度，將較最低稅負制實施前之負向關係幅度為低，即證券交易免稅所得的隱稅仍然存在，然因其租稅優惠減少，使得隱稅成本降低。

The Taiwan government offers various tax incentives to achieve its economic policy goals. However, in the long run, these incentives have seriously eroded tax base and led to unfair tax consequences. To improve the fiscal budget and uphold tax equity, the government enacted the Income Basic Tax Act (IBTA, hereafter), which includes certain specific tax-exempt income items in the tax base. According to the Act, beginning year 2006, a 10 percent minimum tax will be levied on businesses with specific tax-exempt income. This study investigates the impact of IBAT on implicit tax. The results indicate that prior to IBTA implementation, tax-exempt income derived from securities transactions is negatively associated to risk-unadjusted pretax returns on common stocks, consistent with the implicit tax theory. Additionally, the magnitude of implicit tax on tax-exempt income derived from securities transactions is reduced after IBTA implementation.