In the paper, a novel passive vibration isolator is proposed for an ultra-precision sensing system, utilizing a quasi-zero stiffness (QZS) mechanism. The QZS mechanism implements the high static low dynamic stiffness, which effectively reduces the dynamic force transmission while minimizing the static deflection under the natural frequency of conventional passive isolator. Furthermore, it does not need any electric components; the mechanism is suitable for the ultra-precision sensing systems measuring extremely weak electromagnetic fields. However, nonlinear stiffness and hysteresis caused from the negative stiffness elements degrade the system performance. A vertical spring with a pre-tension and eight horizontal plate springs with nonlinear buckling characteristics constitute the proposed system to solve these problems. The mathematical model compares the negative stiffness design with previous QZS research. The buckled plate spring with ball joint design reduces stiffness variation. Transmissibility of the proposed system for low frequency range is investigated experimentally.