A fast in situ spectroscopic ellipsometry technique has been adopted to investigate the plasma-carburising process in a d.c. glow discharge of 100% methane at 1200 K. The optical constant spectra of carbon film on Si at 1200 K have been successfully used for analyzing the plasma-carburising process in a mild steel. A multi-wavelength ellipsometric analysis of the plasma-carburising process in the mild steel strongly suggests that: (1) active carbon transferred to the surface starts diffusing into the mild steel immediately after methane plasma generation and then cementite Fe3C precipitates in a few minutes in the present experimental condition: and (2) carbon diffusion ceases and the cementite dissolves into the bulk or transforms to graphite once graphite begins to be deposited on the surface. In situ ellipsometry is also found to be useful in the surface cleaning process with Ar ion sputtering prior to plasma-carburising in stainless steel. It can monitor the reduction of native oxide on the surface and reveal the timing of plasma-carburising,