Extracellular signal-regulated kinase3 (ERK3) is distinguished from other ERK family members especially in its molecular biological characteristics including the big intron between exons in its gene structure, the serine189 mono-phosphorylated site and C-terminal extention of its kinase structure. The specially activating phosphorylation site of serine189 indicates that all MEKs, which phosphorylate serine/threonine double phosphorylation sites of MAPKs, are unable to activate ERK3. The C-terminal extension involves both subcellular localization of ERK3 and binding to intact cyclin D3, which can profoundly affect cell cycle regulation. According to update reports, ERK3 signal pathway in the regulation of cell cycle might be as follows: Ras→B-Raf→ERK3kinase→ERK3→decrease of CDK compounds of G1-phase→increase of the inhibiting factor (retinoblastoma protein) of S-phase→blockage of S-phase of cell cycle→cell differentiation entry while cell proliferation arrest. Moreover, the activation of ERK3 signaling pathway is also associated with cell differentiation, embryonic development, insulin secretion and cancer diseases.