The tubulization process of amorphous carbon nanopillars was observed in-situ by transmission electron microscopy. Amorphous carbon nanopillars were transformed into graphitic tubules by annealing at about 650^oC in the presence of iron nanoparticles. A catalyst nanoparticle penetrated into an amorphous carbon nanopillar, dissolving it and leaving a graphite track behind. We concluded that the tubulization mechanism is a solid-quasiliquid-solid mechanism where the carbon phase transformation is a kind of liquid phase graphitization of amorphous carbon catalyzed by liquefied metal-carbon alloy nanoparticles.