A total of 365 MKK genes were retrieved from 51 plant species using bioinformatics approaches based on the presence of a conserved S/T-X 5 -S/T domain. Our phylogenetic analyses revealed that group A and B MKKs first appeared in the common ancestors of all green plants, while group C and D MKKs were the rear to arise along with appearance of land or seed plants, respectively, with subsequent divergence of Group E MKKs in flowering plants. It was noteworthy that Group B MKKs were very different compared to those of other groups in many aspects of structure, including exon/intron organizations, phase pattern and NTF2 motif in the MKKs, as well as the biological functions of NTF2 domain with MKK3 will be gradually lost the activity during the evolution despite the loss of this domain maybe affected interaction capability of oneself. The group A MKKs expanded during the evolution, through WGDs followed by diversifications, while group E MKKs expanded during monocots evolution through the ancient tandem duplications. These **data** revealed novel viewpoints about the function and evolution history of plant MKKs.