Abstract: Objective To study the effects of C225 combined with radiotherapy on the proliferation of human gastric carcinoma cell lines MGC803 and BGC823,and to evaluate the mechanisms of the combination effects. Methods (1) The effects of combination treatment on the proliferation of MGC803 and BGC823 cells were evaluated by MTT assay and Q value was calculated to assess the combined effect.(2) Flow cytometry(FCM)analysis was applied to observe the cell cycle distribution and apoptosis of human gastric carcinoma cell lines MGC803 and BGC823 treated by C225 combined with radiotherapy.(3)Western blot was used to detect the expression of EGFR,p-Akt and p-p38MAPK, respectively. Results(1) the combination of C225 and radiation could obviously inhibit the proliferation of MGC803 and BGC823 cells, which showed synergistic effect.(2) Cetuximab alone could induce G₀/G₁ cell cycle arrest of the MGC803 and BGC823 cells,and reduce cells within S phase. When exposed to C225 alone, the percentages of MGC803 cells within G0/G1 phase and S phase were 67.3％and 24.8％，respectively；while the percentages in BGC823 cells were 71.9％and 18.8％，respectively. G^₂/M cell cycle arrest was induced in the cells exposed to radiation alone, and the percentages of G2/M cell cycle arrest in MGC803 and BGC823 cells were 18.2％and 19.5％,respectively. The combination of radiation and C225 resulted in the accumulation of G2/M arrest in MGC803 and BGC823 cells, which were up to 25.7％ and 26.4％ respectively; and also caused a concurrent reduction of cells within S phase, which were 18.6％and 13.5％, respectively. (3)The apoptosis rates of MGC803 and BGC823 cell lines in C225 arm were 4.9% and 9.9%, respectively; and the apoptosis rates were 11.7％ and 19.8％ in radiation arm; both significantly higher than those in the control arm. In the combination arm, The apoptosis rates of MGC803 and BGC823 cell lines were 20.1％ and 47.3％, respectively, significantly higher than that in radiation arm.(4)The combination of C225 and radiation could decrease the expression of EGFR and decrease the activation of its downstream signaling pathway proteins such as p-Akt and p-p38MAPK. Conclusion (1) C225 or X-ray alone could inhibit the proliferation of human gastric carcinoma cell lines MGC803 and BGC823,and exhibit synergistic effect when combined.(2) The mechanism of synergistic effect of C225 in combination with X-ray may be attributed to the inhibition of EGFR signaling pathway, which resulted in: ①the proportion of cancer cells increased in phase G₂/M, and decreased in phase S; ② more apoptosis were induced. Consequently, the proliferation inhibition effect on gastric cancer cells was enhanced.