Abstract: Inerter is a vibration control device which produces resistance related to the relative acceleration at its two ends. Applying inerter to the vibration isolator can increase the inertia, improve the damping ratio of the system, and improve the low-frequency isolation characteristics. The friction of the moving parts of the inerter will produce energy consumption whose influence on the dynamic characteristics of the vibration isolator cannot be ignored. In this paper, the inerter coefficient and apparent friction coefficient of the ball-screw inerter are derived according to theorem of kinetic energy. The inerter force is simplified to the first-order Taylor polynomial, and the range of apparent friction coefficient within which the first-order Taylor polynomial is effective is determined by numerical simulation. Based on the first-order Taylor polynomial of the inerter force, the nonlinear dynamic model and dynamic equation of the parallel inerter isolator considering the friction factor are established which is then solved by averaging method, and the approximate analytical solutions of the amplitude-frequency response and force transmissibility of the inerter isolator are obtained. The effects of apparent friction coefficient and inertia-mass ratio on the amplitude-frequency response and force transmissibility of the vibration isolator are analyzed. Finally, the equivalent damping coefficient of the apparent friction coefficient of the inerter is calculated. The analysis results show that increasing the inerter-mass ratio and the apparent friction coefficient can effectively suppress the resonance peak of the amplitude-frequency response of the vibration isolator and decrease the resonance frequency. Increasing the inerter-mass ratio and apparent friction coefficient will decrease the lower limit of the effective isolation frequency domain of the inerter isolator and the peak value of the force transmissibility and increase the valley value of the force transmissibility. The force transmissibility in high frequency region tends to a stable value. As the inerter-mass ratio increases, the vibration isolation quality in the high frequency region will decrease.