The effect of carbon addition on the grain growth and ordering kinetics of FePt film has been experimentally studied by sputter-depositing a monolithic FePt-20at.%C film of 24 nm. Carbon addition of 20at.% to FePt thin film in a form of FePt (20 nm)/Cn (4 nm) (n = 1, 4) significantly reduced both the grain growth and ordering kinetics. Reducing the thickness of carbon layer, i.e. from n = 1 to n = 4, led to a much finer grain size distribution as well as to a finer grain size. The Monte Carlo simulation study indicated that the decrease of grain growth and ordering kinetics is primarily due to a continuous decrease of the mobility of order – disorder inter-phase with the progress of ordering reaction. This can eventually lead to a stable 2-phase grain structure inter-locked by low mobility inter-phases and is responsible for the formation of a fine grain size distribution in the FePt/Cn film with n = 4.