This paper compares simulation results of a negative impedance converter (NIC) obtained from a circuit-only and circuit-electromagnetic (EM) co-simulation to demonstrate the impact of parasitic effects. The NIC is designed to produce a negative capacitance by cross-coupling two field-effect-transistors. In the circuit-EM co-simulation, the bulk of the transistors and other components are exactly modelled to consider the parasitic effects. The simulations are performed with a 50 ohm termination and a VHF monopole antenna connected at the output port. The results show that the parasitic effects play a significant role in determining the operation frequency of NIC.