Purpose: We evaluate the appearances of the brain metastases (BMs) detected by brain magnetic resonance imaging (MRI) of small cell lung cancer (SCLC) on 18Fluor-Fluorodeoxyglucose (18F-FDG) positron emission tomography/computed tomography (PET/CT). Patients and methods: SCLC patients who had an 18F-FDG PET/CT scan and a brain MRI within 30 days for initial staging were included. MRI was used to detect BMs. The imaging results of BMs on 18F-FDG PET/CT were assessed. On the 18F-FDG PET/CT study, the BMs were classified as undetectable, hypometabolic, hypermetabolic, or mixed patterns (lesions with both hypermetabolic and hypometabolic parts). Results: A total of 51 patients [48 (94.1%) of whom were male and 3 (5.9%) female, with an average age of 62.57 ± 9.64] were included in this study. Fifteen patients (29.4%) were in the limited stage, whereas 36 patients (69.6%) were in the extensive stage. In 11 individuals, MRI indicated 28 BMs. On 18F-FDG PET/CT, 13 of the 28 metastases were visible. The following were the BMs appearances on 18F-FDG PET/CT: hypometabolic (n: 4), hypermetabolic (n: 6), and mixed (n: 3). While the mean diameter of BMs detected in 18F-FDG PET/CT was 16mm; the mean diameter of undetected ones was 4.3 mm. Conclusion: On 18F-FDG PET/CT, BMs can have a variety of appearances, including hypometabolic, hypermetabolic, and mixed patterns. On the other hand, failure to detect millimetric size BMs in 18F-FDG PET/CT prevents proper staging.