The development of computer technology gradually increases. An artificial intelligence-based system also begins to be developed and used in various fields. One product of artificial intelligence is an expert system, used for psychological field. This study aimed to explain and describe the using of expert system to diagnose online game addiction to Junior High School Students. This is based on online game addiction phenomenon happening to Indonesian student especially in Junior High School. The implementation of this expert system used certainty factor method. Steps for developing this system were divided into four, namely designing expert system or architecture of expert system, representing knowledge, designing database, and testing and implementing the system. The results indicate that this system is divided into two domains, including user and admin. The user domain is provided for users who are willing to do online consultations using system expert application. Meanwhile, the admin domain is provided for an admin to manage each datum and question from the user who conducts an online consultation. From 1000 samples, it is obtained that 69% amongst total samples of Junior High School Student have a low-level addiction to online game, 25% experience medium-level addiction, and 6% are highly addicted.