Ubiquitin (Ub) and Ub-like proteins, collectively forming the ubiquiton family, regulate nearly all aspects of cellular processes via post-translational modifications. Studies devoted to specific members suggested a large expansion of this family in plants; however, a lack of systematic analysis hinders the comparison of individual members at both evolutionary history and functional divergence levels, which may provide new insight into biological functions. In this work, we first retrieved a total of 5856 members of 17 known ubiquiton subfamilies in 50 plant genomes by searching both prior annotations and missing loci in each genome. We then applied this list to analyze the duplication history of major ubiquiton subfamilies in plants. We show that autophagy-related protein 8 (ATG 8), membrane-anchored Ub-fold (MUB), small Ub-like modifier (SUMO) and Ub loci encode 88% of the plant ubiquiton family. Although whole genome duplications (WGDs) significantly expanded the family, we discovered contrasting duplication patterns both in species and in subfamilies. Within the family, the ATG 8 and MUB members were primarily duplicated through WGDs, whereas a significant number of Ub and SUMO loci were generated through retroposition and tandem duplications, respectively. Although Ub coding regions are highly conserved in plants, promoter activity analysis demonstrated lineage-specific expression patterns of polyUb genes in Oryza sativa (rice) and Arabidopsis, confirming their retroposition origin. Based on the theory of dosage balance constraints, our study suggests that ubiquiton members duplicated through WGDs play crucial roles in plants, and that the regulatory pathways involving ATG 8 and MUB are more conserved than those controlled by Ub and SUMO.