A novel structured ultra-low frequency nonlinear passive isolator is presented. The isolator is composed of a knife-edge supported sliding beam which offers negative stiffness and a vertical mechanical spring in parallel to get quasi-zero dynamic stiffness. The static characteristics of the isolator are studied. Assuming light viscous damping, the dynamic behavior is investigated and the response under harmonic excitation is derived using a simple approximation. Frequency response curves (FRCs), which exhibit complex double jump phenomenon, are obtained by harmonic balance method (HBM). The limitation of the excitation force level is obtained. The force transmissibility is derived and compared with an equivalent linear system with the same damping ratio. The isolation performance of the nonlinear isolator is shown to outperform the linear system for providing a wider isolation region. The influence of system parameters on the transmissibility is examined and some useful guidelines are given.