Background: Surgical rection is the only potentially curative intervention for locally advanced adenocarcinoma of esophagus, GEJ and stomach. Results from various studies have demonstrated the benefits of perioperative treatment including neoadjuvant and adjuvant chemotherapy or chemoradiation, however, there is lack of universally accepted standard. Recent data demonstrated the benefit of immune checkpoint inhibitor in adjuvant setting in patients who had pre-operative chemoradiation. This single arm phase 2 trial is aimed to evaluate efficacy and safety of pembrolizumab, an immune checkpoint inhibitor, in combination with mFOLFOX in patients with potentially resectable adenocarcinoma of distal esophagus, GEJ and stomach. The primary objective is pathological response rate (ypRR with tumor regression score, TRS ≤ 2). Methods: Newly diagnosed locally advanced (T1N1-3M0 or T2-3NanyM0), potentially resectable adenocarcinoma of distal esophagus, GEJ and stomach by PET, EUS, CT C/A/P and staging laparoscopy were treated with pre-operative mFOLFOX6 (oxaliplatin 85mg/m², Leucovorin 400mg/m², 5-FU bolus 400mg/m², and 5-FU 2400mg/m² infusion every 2 weeks) for 4 cycles and pembrolizumab (200 mg IV q3week) for 3 cycles. Patients with no evidence of metastatic disease by PET and CT C/A/P who are eligible for resection underwent surgery. Post-operative treatment consisted of 4 cycles of mFOLFOX and 13 cycles of pembrolizumab 4-8 weeks postoperatively. Results: Up to 2/10/2021, all 37 patients eligible for the study finished preoperative treatment. 27 had curative intended operations, and all had R0 resection. 5 of 27 (19%) achieved ypCR and 6/27 (22 %) with regression score of 0 in the primary cancer. All except 2 patients (25/27, 93%) had shown pathologic response to the treatment with TRS ≤ 2. 21 patients completed all planned treatment with an average follow-up of 27 months. 2 patients had recurrence/metastatic disease (at 9 and 10 months from the enrollment) with 1 died at 23 months, and the other is still alive at 20 month. The rest patients (19/21) are all free of disease. G3/4 toxicities were reported in 21 of all 37 treated patients. There were no unexpected toxicities. Conclusions: The combination of mFOLFOX and pembrolizumab as peri-operative (pre- and post-operative) therapy in patients with locally advanced adenocarcinoma of distal esophagus, GEJ and stomach is safe. The preliminary benefit data are very encouraging with ypRR of 93%, ypCR of 19 %, and the long-time survival. The data support the combination of chemotherapy and Immune checkpoint inhibitor at perioperative setting. In addition, the data supports the staging laparoscopy for peritoneal disease assessment as the standard in resctacbility evaluation. Clinical trial information: NCT03488667.