Objective: microRNAs (miRNAs) are a new class of non-coding RNAs involved in regulating various biological processes including proliferation, differentiation, and apoptosis, among others. Alterations in miRNA expression are reported in several human cancers, which suggest their potential roles in tumor initiation and progression. Members of the miR-302 cluster are highly expressed in embryonic stem cells (ESCs), where they regulate cell self-renewal and pluripotency. Based on the cancer stem cell (CSC) hypothesis, mis-expression of such genes might contribute to tumorigenicity. This study aims to find a potential link between the expression level of human/homo sapiens miR-302b (has-miR302b) and tumor/grade state of gastric tissues. Materials and Methods: A matched based case-control study was conducted that included tumor and matched marginal non-tumor surgical specimens from 34 patients diagnosed with gastric adenocarcinoma. Realtime reverse transcriptionpolymerase chain reaction (RT-PCR) assays were performed with specific LNA™ primers and SYBR Green master mix. The human embryonic carcinoma cell line, NTERA2 (NT2) and a human gastric adenocarcinoma cell line, AGS, were used as controls. The ability of miR-302b to discriminate tumor from non-tumor gastric samples was evaluated using the area under the receiver operating characteristic (ROC) curve. Results: According to our data, miR-302b expression (normalized to that of the U6 snRNA housekeeping gene) in the pluripotent cell line NT2 was more than 500 times greater than that of the AGS cell line. The level of expression was even lower in tumor and nontumor gastric tissue samples. The data further revealed a down-regulation of miR-302b in gastric tumor samples (p=0.001), particularly in high-grade adenocarcinoma (p=0.009). However, ROC analysis data demonstrated a low sensitivity and specificity of miR-302b expression to discriminate between the tumor and non-tumor state of the samples (AUC=0.63). Conclusion: Despite the upregulation of some hESCspecific genes in tumors, our data revealed a downregulation of miR-302b in high-grade tumors. This data suggested a potential tumor-suppressor role for miR- 302b in tumorigenesis of gastric tissue.