This paper examines the experience effect of judges professionalization on agent's tax litigation performance. We employ the endogenous switching regression model with unknown sample separation to classify experienced judges on income- and business-tax litigation into the professional judges regime. The empirical result shows that Certified Public Accountants (CPAs) perform better than attorneys when the judges lack related tax litigation experiences. On the other hand, we do not find significant performance differences between CPAs and attorneys in the regime of professional judges. The accounting issue subsample test of tax litigation, the professional judges regime sample indicates that CPAs have better litigation performance than attorneys, and also perform advantage in litigation effect and efficiency. In contrast, CPAs perform worst in litigation performance, and both CPAs and attorneys have trail-win rates extremely under the 5% by the judges who lack of experiences. In sum, judges' experience is important for the legal system, and CPAs play a role of professional agent in the tax litigation.