Background: Early tumor shrinkage (ETS) is regarded as predictive marker in metastatic colorectal cancer. In the phase III study comparing oxaliplatin plus S-1 (SOX) to cisplatin plus S-1 (SP) in patients with advanced gastric cancer (AGC), although response rates were almost same between two groups, ETS wasn’t evaluated. Therefore, in AGC, it is controversial whether ETS is predictive or not and which is more suitable regimen, especially as neo adjuvant chemotherapy (NAC). The aim of this study is to compare ETS in SOX with SP and to evaluate the relationship between ETS and clinical outcomes in AGC. Methods: We retrospectively enrolled consecutive 469 patients (SOX 128, SP 341) with histopathologically confirmed HER2 negative AGC treated as 1st-line chemotherapy or NAC in our institution between January 2010 and June 2016. ETS was defined relative change in the sum of the longest diameters of target regions at week 8 (±4) compared to baseline. (Cut-off: 20%). Tumor response was assessed computed tomography using the RECIST 1.1. Patients with peritoneal metastasis and/or bone metastasis without target region were excluded in this study. Results: 192 patients (SOX 60, SP 132) were included in this study. Median tumor shrinkage between SOX and SP were 30% (min-35%, max77%) and 21% (min-89%, max73%), respectively (p= .16). The ratio of ETS between SOX and SP was 53.3% and 50.7%, respectively (p= .75). ETS > 20% was associated with longer OS and PFS when compared with ETS≦20% in SOX group. (ETS > 20% vs≦20%: OS 15.3 vs 10.1 months, HR 0.47 p= .03 PFS 7.6 vs 4.2 months, HR 0.58 p= .09) On the other hand, ETS > 20% was associated with significantly longer PFS only when compared with ETS≦20% in SP group and significant independent predictive factor. (ETS > 20% vs≦20%: OS 15.7 vs 10.9 months, HR 0.79 p= .29 PFS 7.7 vs 4.0 months, HR 0.57 p= .01) In multivariate analysis, ETS remained significant independent predictive factor for PFS in SP group (HR 0.53, p= .006). Conclusions: The ratio of ETS was similar between SOX and SP. ETS may be an early-on-treatment predictor in AGC.