Gas hydrates are a significant energy resource and are usually detected by bottom simulating reflection and submarine geochemical anomalies. Authigenic minerals are related to gas hydrates, with carbonates, sulfates and sulfides being important tracing minerals. Authigenic tubular pyrites were collected from offshore southwest Taiwan in the South China Sea, and were investigated by scanning electron microscopy(SEM) and high-resolution transmission electron microscopy (HRTEM). Authigenic tubular pyrite was composed of framboidal pyrite, within which nanosized graphitic carbon of low crystallinity was discovered. The graphitic carbon coexisted with pyrite and had a texture similar to carbon nanotubes and nanocones, indicating that they likely precipitated from carbon supersaturated C-H-O fluid. Pyrite may act as a catalyst for the conversion of CH 4 to C. The discovery of nanosized graphitic carbon in pyrite indicated it was deposited in sediments that were supersaturated with methane fluid. Thus, nanosized graphitic carbon may be another tracing species for submarine gas hydrates. The discovery of nanosized graphitic carbon deposited in a low temperature environment will enlighten our understanding of the laboratory synthesis and industrial production of graphitic carbon. Gas hydrates are a significant energy resource and are usually detected by bottom simulating reflection and submarine geochemical anomalies. Authigenic minerals are related to gas hydrates, with carbonates, sulfates and sulfides being important tracing minerals. Authigenic tubular pyrites were collected from offshore southwest Taiwan in the South China Sea, and were investigated by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Authigenic tubular pyrite was composed of framboidal pyrite, within which nanosized graphitic carbon of low crystallinity was discovered. The graphitic carbon coexisted with pyrite and had a similar to carbon nanotubes and nanocones, indicating that they are precipitated from carbon supersaturated CHO fluid. Pyrite may act as a catalyst for the conversion of CH 4 to C. The discovery of nanosized graphitic carbon in pyrite indicates it was deposited in sediments that were supersaturated with methane fl Thus, nanosized graphitic carbon may be another tracing species for submarine gas hydrates. The discovery of nanosized graphitic carbon deposited in a low temperature environment will enlighten our understanding of the laboratory synthesis and industrial production of graphitic carbon.