A tension-adjusting device for a chain in a chain saw comprises an adjusting assembly, a fixing base, a brake disk, a position piece and a fixing piece, wherein the adjusting assembly, the brake disk, the position piece and the fixing piece are locked together on the fixing base by a bolt. The adjusting assembly further comprises a knob, a nut, a fixing member, and an adjusting ring. An operator merely needs to rotate the knob of the adjusting assembly to drive the adjusting ring and the brake disk, such that the position piece can be moved forward and backward, thereby to attain the tension adjustment of chain.