The effect of the preformed monolayer on second layer adsorption of N-2 molecules in a graphite slit micropore of pore width = 1.04 - 1.24 nm at 77 K was studied using GCMC simulation. The predominant potential comes from the interaction of a molecule with the preformed monolayer and is more than 70% of the total molecular potential. Therefore, the second stage filling after monolayer adsorption on the micropore walls should he called cooperative filling which was suggested experimentally by Sing et al.