This article discusses the processes of social differentiation of family farmers using the concept of deagrarianization. It aims to identify the occurrence of deagrarianization and agrarianization processes and to analyze the implications of these processes in the family farming differentiation in Salvador das Missões (RS), Brasil. To this end, it proposes the Family Agrarianization Index (IAF) as an summary indicator for these processes. The research was historical and comparative, with primary data obtained from the same group of families, in the years 2003 and 2018. The sample was of the systematic type by community and the data were obtained with a semi-structured questionnaire. The results reveal that the deagrarianization process involves a larger number of families and is more intense than the agrarianization process, demarcating a significant differentiation of families over time. The IAF proved to be effective as an indicator of these processes and introduces into Brazilian literature, in a more systematic way, the perspective of agrarianization / deagrarianization in the analysis of the trajectories of farming families.