Midkine (MK) is reported to be associated with the clinical stages and distant metastases in gastric cancer, and to positively regulate the proliferation of human gastric cancer cells. However, the possible mechanisms of MK in the development of gastric cancer are still not fully clarified. In this study, the therapeutic effect of MK inhibition in gastric cancer in vivo and in vitro was investigated, by knock-down of MK expression with a small interfering RNA (siRNA). MK was expressed in gastric carcinoma tissues and cancer cells. The cytotoxic effect of cisplatin on AGS cells in vitro was attenuated by recombinant human MK, but was promoted by suppressing MK expression via downregulating the Notch signaling pathway-related proteins (Notch 1, Notch 2, Delta‑like 1 and Jagged 1). Suppression of MK expression also promoted the inhibitory effect of cisplatin on AGS cells in vivo. In concusion, suppression of midkine gene promoted the antitumoral effect of cisplatin on human gastric cell line AGS in vitro and in vivo via Notch signaling pathway.