The MAPK/ERK pathway, also known as the Ras-Raf-MEK-ERK pathway, is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell. The pathway includes many proteins, including Mitogen-activated protein kinases(MAPK,originally called ERK), which communicate by adding phosphate groups to a neighboring protein, which acts as an "on" or "off" switch. MAPKs are a highly conserved family of serine/threonine protein kinases involved in a variety of fundamental cellular processes such as proliferation, differentiation, motility, stress response, apoptosis, and survival. At least three MAPK families have been characterized: extracellular signal-regulated kinase (ERK), Jun kinase (JNK/SAPK) and p38 MAPK. The above effects are fulfilled by regulation of cell cycle engine and other cell proliferation related proteins. In this paper we discussed their functions and cooperation with other signal pathways in regulation of cell proliferation. In the MAPK/ERK signaling pathway, the stimulation of tyrosine kinase receptors(RTKs) provokes the activation of MAPKs in a multistep process. Activated Ras activates the protein kinase activity of RAF kinase. RAF kinase phosphorylates and activates MEK (MEK1 and MEK2). MEK phosphorylates and activates ERK. The MAPK/ERK pathway have been shown to play a key role in transduction extracellular signals to cellular responses. MAP kinases lie within protein kinase cascades. Each cascade consists of no fewer than three enzymes that are activated in series: a MAPK kinase kinase (MAPKKK), a MAPK kinase (MAPKK) and a MAP kinase (MAPK). Currenly, at least 14 MAPKKKs, 7 MAPKKs, and 12 MAPKs have been identified in mammalian cells.