Abstract: The magnetorheological (MR) fluid is a kind of novel smart material. MR damper may make full use of superiority of semi-active control system, therefore it has been paid more attention in the field of structural vibration control. Magnitude and amplitude modulation of damping force are two important indexes to evaluate performance of MR damper. Based on mechanical analysis on rheological characteristics of MR damper, a novel MR damper with helix slotted plunger is developed. Through performance experiment and comparison with theoretical analysis, the mechanical performance of MR dampers with various helix slotted plungers is investigated. The results indicate that helix slot may increase friction coefficient on surface of the plunger, and improve the maximum damping force by having no reduction on amplitude modulation of damping force. By changing helix pitch and setting up multiple thread of helix slot, various friction coefficients on surface of the plunger can be achieved. By adjusting helix angle suitably, the reflux of MR fluid may be increased, which may prevent settling of MR fluid.