Gastrointestinal worm infection affects the Sumatran tiger. Due to the lack of tools to diagnose the disease, the Sumatran tigers are very often located in Medan Zoo, North Sumatra, and their handling is very rare. Based on the above problems, we need an application in the form of an expert system by applying the Certainty Factor method which is expected to help in diagnosing Gastrointestinal worm infection in Sumatran tigers. With the existing symptoms, the CF value can be determined to get a diagnosis of Gastrointestinal worm infection. So that it can make it easier for a doctor to treat and diagnose gastrointestinal worm infections in Sumatran tigers. The conclusion obtained from the system, is able to diagnose quickly, precisely and accurately the symptoms of gastrointestinal worm infections and is expected to help the community in diagnosing gastrointestinal worms experienced by Sumatran tigers so that treatment can be carried out immediately. This is an open access article under the CC BY-SA license.