Protein phosphatases are crucial for cell living. Protein phosphatase 2A (PP2A) is an important member of serine/theronine protein phosphatase family, which is involved in almost all biological processes in eukaryotic cell. The 3D structure of PP2A core enzyme and holoenzyme was solved in 2006. These results are significant instructions for us to further understand PP2A structure, interactions between PP2A subunits, and also interactions between PP2A and its binding proteins. In a series of studies, PP2A has been considered as a possible tumor inhibitor, which plays an essential role in tumorigenesis and cell transformation. Subunits composition and crystal structure of PP2A, specific carboxyl-terminal modification of PP2A catalytic subunit, interactions between the three subunits of PP2A, and its biological function as a new tumor inhibitor were summarized.