Gastric cancer is a common gastrointestinal tract cancer. Gastrectomy is the most effective modality of treatment. However, the post-operative course may be complicated with dysfunction of gastrointestinal tract or leakage. If above complications happen, patient and family may have difficulties taking care of drains or wound. By this App, it provides the education of both procedure of gastrectomy and common post-operative complications, record of daily amount and character of drains, daily change of body weight, and uploading pictures of wound and drain fluid. Most important of all, this App is implemented with AI to remind the patients of malnutrition and possible new complication based on the finding of drain fluid.