We have systematically investigated the magnetic properties of (Fe 1Ày Zn y ) 2 Mo 3 O 8 (0 y 1). The ground state of the parent compound, Fe 2 Mo 3 O 8 (y ¼ 0), is an antiferromagnetic (AFM) insulator. The substitution of magnetic Fe 2þ with nonmagnetic Zn 2þ gives rise to ferromagnetic (FM) correlation. With increasing y, the FM correlation evolves and becomes the most enhanced with y ¼ 0:5. One of the possible explanations for the origin of the FM behavior is that Zn 2þ ions prefer the octahedral oxygen coordination to the tetrahedral one. In addition, the magnetizations when y ¼ 0:125 and 0.25 show a stepwise behavior at low temperatures, implying that several magnetic phases are competing with one another.