The purpose of this research is to examines the effect of the audit offer and simultaneous tax services from auditors on the firm value of clients. To test this hypothesis, we used the samples of the auditors’ tax services selected from firms listed on the securities markets of Korea. We use the methodology of regression analysis. The major findings are as follows. First, TAX_SER has a positive relationship with the firm value. The knowledge and experience resulting from the offer of audit services are applied to the process of offering tax services to enhance the professionalism of audit and tax services. Second, the analysis was performed to determine if the type of tax services from auditors and the type of listed markets make any difference in the effect of TAX_SER1 on TQ; the study has revealed that only tax adjustment has a positive effect on TQ in the type of tax services. Depending on the type of listed markets, TAX_SER1 has been found to have a differential effect on the firm value. The results show that investors might decide to invest in those firms where the auditor could increase the firm value through audit and tax services.