The main study topic of this thesis is the Article 99 of the Guidelines for Examination of Profit-seeking Enterprise Income Tax and the range of study is those litigations arising from this same regulation. Generally, the amount of accountingincome accumulated by the financial statements would be equal to the amount of taxable income of profit-seeking enterprise income tax. However, in taxation, the deductible amount of investment loss would be reconciled under the substance-over-form principle so that Tax-Book Difference is therefore resulted. There are many litigations arising from the Tax-Book Difference of investmentloss. To solve these problems, this thesis tries to use ability-to-pat principle as a directive. Then, by means of seeking the constitutional interpretation of the Article 99 of the Guidelines for Examination of Profit-seeking Enterprise Income Tax and economical-finding of its facts, this paper wants to get some feasible solutions to those aforementioned problems.