Introduction. A nonspecific symptom, fatigue accompanies a variety of diseases, including cancer, and can have a grave impact on patients’ quality of life. As for multidimensional instruments, one of the most widely used is the Multidimensional Fatigue Inventory (MFI). This study aims to verify the reliability and validity of the MFI Korean (MFI-K) version. Materials and Method. This study was performed at four university hospitals in the Republic of Korea. Among outpatients visiting the Department of Family Medicine, those complaining of fatigue or visiting a chronic care clinic were enrolled in this study. A total of 595 participants were included, and the mean age was 42.2 years. Results. The Cronbach’s alpha coefficient of the MFI-K was 0.88. The MFI-K had good convergent validity. Most subscales of the MFI-K were significantly correlated with the Visual Analogue Scale (VAS) and Fatigue Severity Scale (FSS). In particular, general and physical fatigue had the greatest correlation with the VAS and FSS. Although the English version of MFI had five subscales, the factor analysis led to four subscales in the Korean version. Conclusion. This study demonstrated the clinical usefulness of MFI-K instrument, particularly in assessing the degree of fatigue and performing a multidimensional assessment of fatigue.