Directorate General of Tax as a public organization has to implement tax administration reform in order to improve the service quality to taxpayer. For that purpose, Foreign Investment For Tax Office has applied Modern Tax Administration System since the year 2004. This research is meant to know how significant is the influence of Moern Tax Administration System at Foreign Investment Four Tax Office to service quality. Modern Tax Administration System is an implementation of the tax adminsitrative reform that is defined as the deliberate use of authority and influence to apply new measures to an administrative system so as to change its structure procedure, strategy, and culture to improve public sector performance. Meanwhile, service quality id defined as a stake holder value regarding the service dimension whch is perceived as positive value acquirement to the degree of satisfaction. It is measured by ten dimensions that is tangibel, reliable, responsiveness, competence, courtesy, credibility, security, acess, communication, and understanding the customers. This research is carried out by using an explanation method which explaining a causal relationship between independent variables and a dependet variable. It uses a quantitative approach. Data is collected with survey by giving questionnaires to all officer of Foreign Investment Four Tax Office. Then the data is analysed with correlation and regression. The result of this analysis indicate that there is a positive strong relationship between Modem Tax Administration System together which consist of organization chart modemization, organizational procedure modernization, organizational strategy modernization, and organizational culture modernization and service quality. All sub variables of the Modern Tax Adrninistration System have significant influences to service quality of Foreign Investment Four Tax Ofiice. The research proposes that Foreign Investment Four Tax Office should improve Modem Tax Administration System higher either organization chart, organizational procedure, organizational strategy, and organizational culture to support the service quality using approaches: making a job description more detail to prevent duplication, making an easier access of getting new regulations from the intemet or intranet, promoting knowledge and skill of the employees through in house training, out bond training, and improving moral and ethics of the employees through ?kultum" and Emotional Spiritual Quotient (ESQ) training.