The nucleoprotein (N) of non-segmented negative-sense RNA viruses such as Measles, Ebola and Newcastle disease viruses (NDV) enwraps viral genomic RNA and assembles into helical filaments, serving as the template for replication and transcription. Using cryo-Electron Microscopy, we identified and resolved NDV N forming a 4.8 ? resolution clam-shaped structure with two single-turn spirals packing in a back-to-back mode, which have not been reported before. More interestingly, this clam-shaped structure can function as a seed to assemble into double-headed filaments with two separate RNAs inside. Disruption of clam-shaped structure by transition mutation on loop interface (residues 114-120) yields a single-headed filament and exposes RNA 5’ end accessible to nuclease, which will abolish viral genome replication in minigenome analysis. Our data manifest a self-coating mechanism in which clam-shaped structure and derived double-headed filaments fully protect NDV viral genome and are new potential targets for drug design.