Well-known marks are intellectual works that have economic value and have created their own perspective in society so that they become assets for a company in the trading world, therefore well-known marks must be protected. In practice, a brand with reputation (well-known mark) is commonly imitated and registered in bad faith by unauthorized parties simply because the mark has not been registered in Indonesia. The purpose of this paper is to comprehensively examine the application of constitutive principles in the case of cancellation of registered trademarks that are applied in bad faith and to analyze the judges' considerations in deciding cases of cancellation of registered trademarks in bad faith in the decision of the Commercial Court Number 10/Pdt.Sus-Merek/2019/PN.Niaga.Jkt.Pst. This research is a normative legal research supported by empirical studies. The approach used in this study is based on a statutory approach and a case approach. The legal materials used in this study are primary legal materials namely regulations that are related to trademarks and secondary legal materials in the form of legal books and journals. Moreover, this research also uses non-legal materials that contain legal information obtained through interviews with legal practitioners in the field of Intellectual Property Rights. Associated with the basic principle of Law No. 20 of 2016 namely the constitutive principle (first to file), legal protection of trademarks in Indonesia can only be obtained by the first registrant in good faith. For trademarks registered in bad faith, their registration may be cancelled. Based on the judge's decision in the Commercial Court Decision No.10/Pdt.Sus- Merek/2019/PN/Niaga.Jkt.Pst, this study does not agree with the Panel of Judges who rejected the plaintiff's claim and declared the defendant as the rightful trademark holder.