Implementation of Law no. 5 of 1999 is quite important to be implemented using circumstantial evidence. This is due to a characteristic that comes from the law in conducting the business competition and acts in anti-competition. However, it is unfortunate that there is uncertainty in the law and controversy when using circumstantial evidence. This study will examine how circumstantial evidence will be used to enforce the law on business competition in Indonesia. This study will use a normative legal study method with a statutory, conceptual, and case approach. The study data fully uses secondary data, both in primary, secondary, and tertiary legal materials collected through library research or document studies and analyzed using abstractive-interpretative-qualitative analysis methods. This study found that law enforcement against Law no. 5 of 1999 through circumstantial evidence (circumstantial evidence) is still characterized by legal uncertainty. The use of circumstantial evidence is very necessary to enforce business competition law. The special characteristics of business competition law and business competition cases cause business competition law enforcement to be illogical if it only relies on the proof process through hard evidence or direct evidence. Therefore, it is necessary to improve Law no. 5 of 1999 is related to the existence of the circumstantial evidence, either by explicitly acknowledging circumstantial evidence as valid evidence or by including it explicitly as part of the guiding evidence.