The effect of carbon addition on the grain growth and ordering kinetics of CoPt film has been studied experimentally by sputter-depositing a monolithic CoPt-20 at.%C film of 24 nm. The carbon addition of 20 at.% to CoPt thin film in the form of CoPt (20 nm)/C-n (4 nm) (n = 1, 4) significantly reduced both the grain growth and ordering kinetics. Reducing the thickness of the carbon layer, i.e. from n = I 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 the order-disorder inter-phase with the progress of the ordering reaction. Reduction of the interphase mobility can eventually lead to a stable two phase grain structure inter-locked by low mobility interphases and is responsible for the formation of a fine grain size distribution in the COPt/C-n film with n = 4.