MgB2 thin films were deposited by e-beam on Al2O3(001)substrates. Resistive transition of the MgB2/Al2O3 was investigated in different magnetic fields applied parallel or perpendicular to the ab plane of the films by the standard four-probe method. An activation energy model is suggested to analyze quantitatively the activation energy of flux lines and resistive transition in whole transition temperature range. The anisotropy parameter γ=Hc2ab(0)/Hc2c(0)=2.26 was obtained by analyzing the upper critical field using the polynomial Hc2(t)=Hc2(0)+At+Bt2.