Protein phosphatase 4 (PP4) is an important protein serine/threonine phosphatase belonging to PP2A family. Previous study showed that protein phosphatase 4 played an important role in centrosome maturation in D. melanogaster and C. elegans. But as an evoulutional conserved phosphatase, little was known on its function in mammalian cells. siRNA expression vector was used to suppress PP4 expression in A549 and clonal cell lines was obtained in which the expression of PP4 was inhibited. The morphology, growth and mitosis process of the cells were investigated. It was found that cell growth and proliferation were greatly inhibited and many cells became multinuclei comparing with the control cells. Further study showed that the mitosis and cytokinesis became abnormal in PP4 silencing cells. It was proposed that PP4 played an important role in mitosis and cytokinesis. PP4 expression suppression resulted in multinucleus, then inhibited the proliferation of cells.