We investigated the crystallization behavior of amorphous carbon (a-C) in a-C/Co/a-C trilayer (C: 650 Å: Co: 70-210 Å thick) thin films using in situ transmission electron microscopy (TEM), differential scanning calorimetry (DSC) and Raman spectroscopy. In situ and ex situ TEM showed that a-C, in contact with cobalt, transforms into polycrystalline graphite upon heating above 500°C. The heat of crystallization, as measured by DSC, is 19 ± 2 kJ/mol-C. The Raman spectroscopy also confirmed Stokes scattering peaks at 1580 and 1360 cm-1, indicating the polycrystalline nature of the graphite. We did not observe any metastable cobalt-carbide phase formation during the reaction (as occurs in co-sputtered Co-C film crystallization), but our cross-section TEM revealed the presence of the face centered cubic (f.c.c.) Co phase between the a-C and graphite. This observation suggests that the crystallization or graphitization of the a-C is mediated by the Co phase, similar to the behavior in other eutectic metal-metalloid systems (e.g. Al-Si, Ag-Ge).