Objectives: The aim of this study was to evaluate 18F-fluoro-2-deoxy-glucose (FDG) uptake patterns in primary tumors and metastatic lesions, and also to assess the diagnostic contribution of positron emission tomography/computed tomography (PET/CT) in the initial staging of gastric cancer (GC).Methods: The total number of 341 patients with GC were included in this study. All 18F-FDG PET/CT imagings were performed for initial staging. The maximum standardized uptake value (SUVmax) of primary tumor, obtained from 18F-FDG PET/CT imaging was compared between subtypes of GC.Results: Mean SUVmax of 339 patients’ primary tumor was 12.9±8.6. The highest mean SUVmax was detected in patients with medullary subtype GC (17.8±9.9) while the lowest mean SUVmax (9.7±7.6) was seen in signet ring cell carcinoma (SRCC). The primary mean SUVmax was found statistically higher in adenocarcinoma (AC) group than SRCC group (p<0.001). Higher SUVmax values were found statistically significantly correlated with advanced age (aged ≥60) and increased tumor size (>3 cm) in patients with AC (p=0.03). Primary tumor SUVmax was found statistically higher in regional lymph node (RLN) positive patients than in RLN negative patients in AC and SRCC groups (p<0.001 and p=0.012, respectivelly). Also, in patients with SRCC, SUVmax was significantly higher in the distant metastatic group than in the group without metastasis (p=0.025).Conclusion: Increased primary tumor SUVmax was associated with some of clinical parameters such as age and RLN metastasis in patients with AC. However, there was no relationship between distant metastatic state and primary tumor 18F-FDG uptake in AC. However, high SUVmax of primary tumor in SRCC was associated with regional and distant metastasis, and primary tumor 18F-FDG uptake may be a prognostic value for this subgroup.