The cyclo[18]carbon was synthesized experimentally for the first time in August 2019 [K. Kaiser et al., Science 365, 1299 (2019)]. The semiconductor property of the cyclo[18]carbon makes it possible to become an essential element of molecular electronic devices. In our work, the electron transport properties of zigzag graphene nanoribbon cyclo[18]carbon-zigzag graphene nanoribbon junction [(8-ZGNR)-C18-(8-ZGNR)] were studied, using the non-equilibrium Green’s function approach based on the density functional theory. The coplanar and perpendicular configuration constitutes the current ON and OFF state, respectively, with the high ON/OFF ratio approaching to 104. A significant negative differential conductance (NDC) with a peak-to-valley ratio of 1.5 in the bias range of 0.3 V–0.4 V was found. The NDC arises due to the resonance of the edge state of ZGNRs and the frontier orbitals of C18 within the bias window. Moreover, the high current ON/OFF ratio and NDC are robust against variations in the lead’s width, length of short-carbon chain, and the bridging way of C18 on leads. Specifically, the Ipeak/Ivalley of the coplanar configuration with the C5 spacer can increase to 5.7 in the bias range of 0.4 V–0.5 V. Our computational studies may guide design of multi-functional molecular devices based on C18.