The Ras family genes are proto-oncogenes that are highly conserved from Drosophila to humans. In Drosophila, Ras V12 is a constitutively activated form of the Ras oncoprotein, and its function in cell-cycle progression is context dependent. However, how it influences the cell cycle of female germline stem cells (GSCs) still remains unknown. Using both wild-type GSCs and bam mutant GSC-like cells as model systems, here we determined that Ras V12 overexpression promotes GSC division, not growth, opposite to that in somatic wing disc cells. Ras performs this function through activating the mitogen-activated protein kinase (MAPK) signaling. This signaling is activated specifically in the M phase of mitotic germ cells, including both wild-type GSCs and bam mutant GSC-like cells. Furthermore, Ras V12 overexpression triggers polyploid nurse cells to die through inducing mitotic stress. Given the similarities between Drosophila and mammalian GSCs, we propose that the Ras/MAPK signaling also promotes mammalian GSC division.