Background: The frequency of microsatellite instability-high (MSI-H) phenotype in sporadic gastric cancers (GCs) widely varies from 8% to 20%. From prior data, MSI-H phenotypes are significantly associated with female sex, older age, antral location of the tumor, well to moderate differentiation, intestinal type, non-signet ring cell component, mucinous histologic type, a moderate to severe lymphoid stromal reaction, and a lower TNM stage. This study investigated the proportion and distribution of PD-L1 expression of MSI-H GCs compared to microsatellite stable (MSS) GCs. Methods:The study consists of 847 cases of unselected GC. MSI analysis was performed on all GC cases by immunohistochemistry for mismatch repair proteins followed by multiplex polymerase chain reaction. Among them, 72 GCs were found to be an MSI-H phenotype. After matching the T category of GCs, 200 cases of GC with MSS phenotype were selected for a control group. Immunohistochemistry for PD-L1 (SP263) was performed on 72 MSI-H GCs and 200 MSS GCs. Results: Patients with MSI-GC were significantly associated with advanced age (p <0.001), advanced tumor (p=0.0173), antral location of the tumor(p=0.0018), and T2 and T3 category (p=0.0031) but not T1 or T4 category. Histologically, MSI-H GCs occurred frequently in ulcerofungating mass (45.8%, p < 0.0001)), intestinal type (73.6%, p=0.0102), well to moderately differentiated tumor (56.9%, p=0.0102), tubular histologic type (66.7%, p=0.0013), tumor necrosis (18.1%, p <0.0001), and peritumoral neutrophilic infiltrate (40.3%, p <0.001) compared to those of 200 MSS GCs. In the evaluation of PD-L1, 55 (76.4%) MSI-H tumors revealed PD-L1 expression with ≥ 1 CPS. The predominant PD-L1 staining was found in intratumoral and peritumoral immune cells (66.7%) than within tumor cells (9.7%). The distribution of positive immune cells was predominantly located in the periphery of the tumor (37.5%) than within the tumor (9.7%). According to the criteria for PD-L1 positivity as at least 10 CPS, approximately 50% of MSI-H GCs were considered PD-L1 positive tumors with at least 10 CPS. Conclusions: In this study, the prevalence of MSI-H phenotype is approximately 8.5% of unselected GCs. Approximately 76% of patients with MSI-H tumors showed PD-L1 expression with at least 1 CPS, and 50% revealed PD-L1 expression with at least 10 CPS, who can respond better to immune checkpoint inhibitors with promising therapeutic benefits. Due to the peritumoral expression of immune cells in PD-L1 positive GCs with MSI-H phenotype, different endoscopic approaches to obtain tumor tissues for PD-L1 evaluation would be considered, such as sampling tissues at the tumor periphery.