Recently [1] an unusually strong field dependence of the activation energy, U(B), of magnetic flux motion in the novel superconductor, MgB2 , was found. The aim of the present work is the definition of the dependence U =f(B). In Fig.1 is shown the field dependence U(B), found for thin MgB2 films in [1]. For choosing of the type of the function U(B) the calculating program was wrote in „Delphy-language”. The working algorithm was the method of least squares.figuraFig.1. Magnetic field dependence of the activation energy of flux flow in MgB2 deposited on Al2O3 substrate. The data are taken from [ 1]. As one can see, MgB2 demonstrates a strong field dependence of the activation energy, the exponential one, U(B) = 11894e-0.37B, with the critical index equal to 0.37.