地上經濟活動逃稅行為與地下經濟活動逃稅行為不同，地上經濟活動逃稅指人們指從事政府核備的生產以及消費活動，但是規避法律規定應繳納的租稅；地下經濟活動指人們所從事的經濟活動並未經過政府事先核備，故又稱灰色經濟，從事地下經濟活動主要目的在於避免被稅捐單位課稅。本文主要分兩大部分，第一部分設定一個同時納入地上經濟與地下經濟之效用函數，並將公共財依地上與地下經濟權重區分，探討納稅人地下經濟所得申報與租稅遵循率之間的關係，並在查到被處罰的機率條件下分析納稅人地下經濟所得的最適參與率。第二部分探討稅率提高是否造成地下經濟勞動人口比例上升，最後發現主要關鍵因素決定於納稅人財富或所得增加後對於相對風險趨避程度的態度。

Ground economic activities differ from underground economic activities on taxpayer's Behavior. The tax evasion manner of ground economic activities interpreted as people engaged in production and consumption activities whereas government consented to, but avoided taxation which shall collect. Thus, underground economic activities, which known as gray economy, mean people engaged in economic activities without prior permission from tax collector. Individuals engaged in underground economic activities mostly aimed at passing the burden of taxation by tax collector. In this paper, we assume the basic assumption of the model through a log ground and underground utility function. Again extending audit costs, concealed costs and public goods into our system. Under our assumption, we derive the optimal relationship between representative individual's evasion and tax rate. Furthermore, examine optimal income reporting and tax compliance problem .Finally, we show that whether a increase in tax rate caused the underground economic activities increase in the proportion of the labour force, mainly determined by the representative taxpayer's attitude to avoid increasing relative risk levels of wealth. Thus the government is more likely to be able to use underground economic market to implement optimal tax scheme.