Abstract: The cases-choice for tax audit contains two main aspects: the first is to distinguish whether the taxpayer is tax evasion, the second is to estimate the quantum of tax evasion. The paper systematically have studied the estimating of the quantum of tax evasion and made progresses and results. The author first applied the Tobit model to estimate the quantum of tax evasion and fairly solved the problems in the estimations of the quantum of tax evasion.