Abstract: Ubiquitin (Ub) is a conserved small protein consisted of 76 amino acid residues. Ubiquitin-dependent proteolytic pathway is the most important selective proteolytic pathway. Ubiquitin system is composed of ubiquitin, ubiquitin-activating enzymes, Ubiquitin-conjugating enzymes, ubiquitin-protein ligases, ubiquitin-C-terminal hydrolase and 26S proteasomes. This review discussed and described components of ubiquitin system, structure and function of each component, machanisms of protein ubiquitination and degradation of ubiquitinated proteins, and modes of recognition of substrates.