We study the effect of electron interaction on the spin-splitting and the $g$-factor in graphene in perpendicular magnetic field using the Hartree and Hubbard approximations within the Thomas-Fermi model. We found that the $g$-factor is enhanced in comparison to its free electron value $g=2$ and oscillates as a function of the filling factor $ u $ in the range $2leq g^{ast}lesssim 4$ reaching maxima at even $ u $ and minima at odd $ u $. We outline the role of charged impurities in the substrate, which are shown to suppress the oscillations of the $g^{ast}$-factor. This effect becomes especially pronounced with the increase of the impurity concentration, when the effective $g$-factor becomes independent of the filling factor reaching a value of $g^{ast}approx 2.3$. A relation to the recent experiment is discussed.
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