The objective of this research is to show how interaction quality between audit committee and internal auditor can influence the internal audit function effectiveness. The interaction between audit committee and internal auditor can be in the form of meetings between audit committee and internal auditor, involvement of audit committee in appointing and dismissing the chief of internal audit unit, and the involvement of audit committee in internal audit program and process development. The quality of this interaction should increase if at least the meetings are more frequent and the involvement of The audit committee to the aforementioned activities are more intense. This research is conducted by interviewing The audit committee members and the chief of internal audit unit in Bank XYZ.  We confirm the result by distributing questionnaires to The audit committee members and internal audit function staff. The findings of this study is that the frequency of meetings between audit committee and internal auditor, audit committee involvement in appointing and dismissing the chief of internal audit unit, and audit committee involvement in internal audit program and process development have some influence on the internal audit function effectiveness.