ABSTRACT The objective of this study is to examine the effects of internal audit proficiency on firm goal achievement. Internal audit proficiency consists of continuous internal control assessment, integrative internal audit planning, best internal audit practice, dynamic internal audit review and excellent internal audit reporting. In this study, 105 Thai-Listed firms are the sample. With the results of this research, continuous internal control assessment has a significant positive effect on financial reporting reliability. Internal audit planning has a significant positive effect on organizational value increase and risk management efficiency. Best internal audit practice has a significant positive effect on operational effectiveness enhancement. Also, excellent internal audit reporting has a significant positive effect on financial reporting reliability and operational effectiveness enhancement. Furthermore, organizational value increase and risk management efficiency have a significantly positive effect on firm goal achievement. Meanwhile, financial reporting reliability and operational effectiveness enhancement have no impact on firm goal achievement. Moreover, governance vision has significantly positive effect on continuous internal control assessment, integrative internal audit planning, best internal audit practice and excellent internal audit reporting. Internal audit competency has a significantly positive effect on integrative internal audit planning, best internal audit practice, dynamic internal audit review, and excellent internal audit reporting, while internal audit learning has a significantly positive effect on integrative internal audit planning and best internal audit practice. However, future research is needed to collect in-depth data so as to reflect the actual concept of internal audit proficiency of Thai-listed firms in Thailand. Likewise, future research is strongly recommended to collect data from a larger and/or comparative population, or from other auditing professions such as certified public accountants (CPAs), tax auditors (TAs), governmental auditors (GAs), and co-operative auditors (CAs) in Thailand or elsewhere. Keywords Internal audit proficiency, continuous internal control assessment, integrative internal audit planning, best internal audit practice, dynamic internal audit review, excellent internal audit reporting, financial reporting reliability, operational effectiveness enhancement, organizational value increase and risk management efficiency, governance vision, internal audit learning, internal audit competency