Abstract: ObjectiveTo investigate the expression of S100P on RNA and protein levels in relation to clinicopathological features with gastric cancer patients. Methods Expression of S100P in primary gastric cancer (n=93) was analyzed using immunohistochemistry.Then, immunoblotting and RT-PCR were conducted on part of immunohistochemical-positively stained gastric cancer tissues and matched gastric mucosas. Results The immunohistochemical staining for S100P was predominantly nuclear with some cytoplasmic staining in normal gastric mucosa.As a result, 52.7% (49/93) of primary tumors and almost all the normal gastric mucosa were stained positively and S100P showed down-regulated expression in gastric cancer than in normal gastric mucosa.Further analysis with clinicopathologic data showed that the relative higher S100P expression strongly correlated with tumor infiltration depth (P=0.006) and tumor size (P=0.001).Meanwhile, the expression of S100P between RNA and protein level (results from immunohistochemistry) was closely correlated (P=0.024).But it can not be used as a dependent prognosis factor. Conclusion S100P was down-regulated in gastric cancer；S100P expression correlated with tumor infiltration depth and tumor size；Down-regulated expression of S100P can be used as a supporting marker for prognosis prediction.