The adsorption of NCCN molecule onto the outer surfaces of pristine and Cu-decorated Si12C12 (Cu@Si12C12) nanocage is investigated using density functional theory (DFT) calculations. According to the results of the calculations, the NCCN molecule could not strongly adsorb on the exterior surfaces of Si12C12 nanocage. Therefore, in order to increase the efficiency of Si12C12 nanocage for NCCN adsorption, Cu decorating process was investigated. The results show that interaction between the NCCN molecule and Cu@Si12C12 is chemisorptions with appreciable adsorption energy. Therefore Cu@Si12C12 is a suitable adsorbent and it is expected that can potentially to be used as nanosensors for detecting the presence of toxic NCCN molecule.