This paper is focused on interaction between two closed-connected high-voltage DC (HVDC) lines. This interaction is studied by employing electromagnetic transients (EMT)-based nonlinear modeling in MATLAB R /Simulink R software environment. In order to describe the mechanism behind the HVDCs interaction, both nonlinear time-domain simulations and modal analysis of the coupled HVDC links, using linearized version of the system, have been performed. System stability has been assessed and the path of interactions has been identified by computing the participations of various states in the oscillatory modes.