Case 1: A male patient in his 60's who had rectal cancer and liver metastases underwent Hartmann's operation in January 2009. In April 2009, he received chemotherapy with modified FOLFOX6 plus bevacizumab as second-line treatment. Thirteen days later, he complained of abdominal pain and visited the emergency department. Computed tomography(CT)revealed gastrointestinal tract perforation. He underwent emergency surgery. However, dirty brownish ascites was observed, and the perforation point of the gastrointestinal tract could not be found. We suspected perforation of the colon and created an artificial anus in the terminal ileum. After the surgery, his condition improved. Case 2: A male patient in his 60's who had rectal cancer underwent Miles' operation in March 2005. In February 2010, CT revealed local recurrence in the presacral region. After radiotherapy, he received chemotherapy with CapeOX plus bevacizumab. During the course of the chemotherapy, he was admitted for upper abdominal pain and fever. He was diagnosed with a perforation of the transverse colon and underwent emergency surgery. After the surgery, drainage was needed for peritonitis, for about 1 month. At 93 days after the surgery, he was discharged. Gastrointestinal tract perforation is one of the most serious adverse events associated with bevacizumab. To avoid death caused by serious adverse events, medical staff and patient orientation or education on the possible serious adverse event is very important.