A quasi-zero stiffness (QZS) isolator is configured by combining an Euler buckled beam negative stiffness corrector and a linear isolator. Making the load balanced at the zero stiffness point, the QZS isolator can offer ultra-low frequency vibration isolation performance. However, overload or underload usually happens in practice. This paper presents the effect of load imperfection on the performance of the QZS isolator by defining force and displacement transmissibility and the result of which is compared with a linear isolator. Assuming slightly overload or underload and using screw to adjust the actual load to the zero stiffness position for the QZS isolator, the dynamic response of the vibration isolation system is obtained by using HBM. For linear isolator, the resonance frequency will decrease and the peak transmissibility will increase if there is an overload. In the case of force transmissibility, the QZS isolator is similar to the linear isolator. However, the result is opposite turn to the displacement transmissibility. Increasing load makes both the peak and resonance frequency of the absolute and relative displacement transmissibility shifts to the higher value. The results present here can be a useful guideline when design such kind of vibration isolator.