We apply the spin-polarized free-electron model to theoretically study the tunneling magnetoresistance (TMR) in a ferromagnet-metal-insulator-ferromagnet tunneling junction. In the presence of the metallic layer, the tunneling of electrons for current flow through the junction can be divided into two components: the coherent tunneling and the sequential tunneling components. Our calculations show that the attenuated TMR oscillation is well fitted by a damped oscillation function with an exponential decay if only the coherent tunneling is considered. But, if the sequential tunneling is included in the calculations, the behavior of the attenuated TMR oscillation slightly deviates from the damped oscillation function. (C) 2011 Elsevier B.V. All rights reserved.