The research aims to formulate a substantive theory on Indonesia's Large Family Businesses on preserving multi-generational family business sustainability utilizing classic grounded theory methodology. This study focuses on two main research questions, which are (1) what are the Indonesian large-family business Founders' main concerns for preserving, perpetuating, and managing multi-generational large family businesses beyond the third generation, and (2) how they address, resolve, and manage this main concern. Although little research exists regarding the Indonesian large family business and its dynamic, all large family businesses in Indonesia must begin to consider passing their baton to the third generation in order to preserve their family dynasty. This study uses a classic grounded theory methodology involving a purposive sampling group of 27 respondents as follows: the founders or the patriarch (also known as the first generation), the descendants or the successors (also known as the second-generation), the third-generation members of the immediate or extended family, and other family members or acknowledged non-family member executives or formal executives. They are dispersed from four conglomerate groups with significant impact on the economy, who were interviewed through online video conferencing due to the COVID-19 pandemic. The data were analyzed using open coding, sorting, memoing, constant comparative analysis, selective coding, and theoretical sampling. This study generated the Theory on Inter-generational Perpetuation of Large Family Businesses that addressed the participants' main concern in this study: perpetuating the business. This Theory on Inter-generational Perpetuation of Large Family Business, also called the “PHC Triangle”, is the conceptualization of the preparation of the founders or parents of the large family business in Indonesia with the aim to transfer the family business to a later generation in order for the family to continue the business and involvement of family members. The key finding of this research is that the primary concern of perpetuation lies within parenting, harmonizing, and collaborating. Firstly, parenting is done with the intention to equip, foster, mould and cultivate the next generation with the soft elements of the process: modest and robust character, embracing founders' compelling purposes, adopting family values, embracing education, gaining outside experience. Secondly, a family business must fully understand that harmony amongst family members is crucial, and to accept that one individual is not capable of handling every aspect of a business alone. As early as possible, the family is required to formalize the family constitutions to regulate family governance and involvement. Thirdly, a family must be willing to collaborate with non-family members, executives, or employees. Furthermore, this study lays down the Seven Stepping Stones of Inter-generational Perpetuation for the preparation and implementation of the family business. The Theory on Inter-generational Perpetuation of Large Family Businesses enhances the Sustainable Family Business Theory (SFBT) to facilitate the correct response to disturbances in family-business transactions in order to achieve long-term sustainability, leading to inter-generational perpetuation. The Theory on Inter-generational Perpetuation of Large Family Business, or the “PHC Triangle”, is proposed as an intervening variable to the sustainability of large family business resulting in the successful perpetuation of the family business. This study constructs a valuable contribution to the study of the large family business and family business in general. Conclusively, it ascertain the more robust option that the survival of family business over several generations is conceivable with the Theory on Perpetuating Inter-generational of Large Family Business The Importance of Parenting to Equip, Harmonizing to Prosper, and Collaborating to Equip.