Purpose This study was performed to assess the usefulness of $^{18}F$-fluorodeoxyglucose ($^{18}F$-FDG) positron emission tomography (PET) or PET/computed tomography (CT) for distinguishing thymic epithelial tumors according to World Health Organization (WHO) classifications. Methods We analyzed a total of 45 patients (range, 29. 75 years of age; mean, 55 years) with pathologically confirmed thymic epithelial tumors who underwent pretreatment $^{18}F$-FDG PET or PET/CT between November 2003 and October 2009. The size, visual grading of uptake value, peak standardized uptake value (SUVpeak), uptake pattern, and contour of each tumor, and associated findings on PET or PET/CT, were analyzed relative to the three simplified WHO subgroups: less-invasive thymomas (types A and AB), more-invasive thymomas (types B1, B2, and B3) and thymic carcinomas. We statistically assessed the relationship of $^{18}F$-FDG PET or PET/CT findings with these simplified subgroups. Results Of the 45 patients, ten had less-invasive thymomas, 23 had more-invasive thymomas, and 12 had thymic carcinomas. The SUVpeak of the less- and more-invasive thymomas were significantly lower than those of thymic carcinomas (p<0.000), but there was no difference in SUVpeak between less- and more-invasive thymomas. The visual grading scale (p<0.000), uptake pattern (p=0.001), and contour (p<0.000) of the tumors differed significantly among the three simplified subgroups. Conclusion The image findings of $^{18}F$-FDG PET or PET/CT differed significantly by histologic subgroups. Pretreatment evaluation with $^{18}F$-FDG PET or PET/CT might be helpful in differentiating subgroups of thymic epithelial tumors.