Purpose: Reoperations after gastrectomy for gastric cancer are performed for many types of complications. Unexpected reoperations may cause mental, physical, and financial problems for patients. The aim of the present study was to evaluate the causes of reoperations and to develop a strategic decision-making process for these reoperations. Materials and Methods: From September 2002 through August 2010, 6,131 patients underwent open conventional gastrectomy operations at Samsung Medical Center. Of these, 129 patients (2.1%) required reoperation because of postoperative complications. We performed a retrospective analysis of the patients using an electronic medical record review. Statistical data were analyzed to compare age, sex, stage, type of gastrectomy, length of operation, size of tumor, and number of lymph node metastasis between patients who had been operated and those who had not. Results: The variables of age, sex, tumor stage, type of gastrectomy, length of operation, and number of lymph node metastases did not differ between the 2 groups. However, the mean tumor size in the reoperation group was greater than that in the non-reoperation group ($5.0{pm}3.7$ [standard deviation] versus $4.1{pm}2.9$, P=0.007). The leading cause of reoperation was surgical-site infection (n=49, 0.79%). Patients with intra-abdominal bleeding were operated on again in the shortest period after the initial gastrectomy ($6.3{pm}4.2$ days). Patients with incisional hernia were not reoperated on until after $208.3{pm}81.0$ days, the longest postoperative period. Conclusions: Tumor size was the major variable leading to reoperation after gastrectomy for gastric cancer. The most common complication requiring the reoperation was a surgical site-related complication.