Hereditary nonpolyposis colorectal cancer (HNPCC) is the most common hereditary colorectal cancer syndrome and accounts for about 5% of colorectal cancer. It is inherited as autosomal dominant type and is caused by germline mutations in mismatch repair genes such as MLH1, MSH2, MSH6, and PMS2. Patients with HNPCC are characterized by a high level of microsatellite instability. They commonly develop colorectal cancer at young age and increase risk of extra-colic malignancies, especially endometrial cancer. They also show better oncologic outcomes compared to sporadic colorectal cancer. Several tools are used in diagnosis of HNPCC, including history taking, microsatellite instability test, immunohistochemistry for mismatch repair protein, and gene test. Affected patients and their families should get genetic counseling and regular surveillance for cancers, which can improve their survival rate.