In this paper authors propose a liquid inertia damper that has a screw shaped piston in order to have effect of a series inertia mass. The damper consists of a piston and a cylinder, a screw shaped piston. Water is filled in the cylinder. The peripheral gap of the screw can act as an orifice of a by-pass pipe, and an inertia force is produced by a liquid flow through the gap. In order to confirm the inertia force, a test damper is manufactured, and resisting force characteristics are measured. It is apparent from test results that the damper can have a sum of the inertia force and nonlinear damping force. The series inertia mass is depended on a relation between a diameter of the cylinder and a gap of the screw. To confirm the vibration reduction, harmonic frequency response of one-degree-of-freedom system with the liquid inertia damper is calculated. Finally, the effects of vibration suppression are confirmed numerically.