One effective method to improve electronic conduction of sol-gel Al-doped ZnO films is obtained by deductions from near-infrared transmittance spectra based on free electron absorption theory. By modifying the preheating procedure, the grains of Al-doped ZnO films become smaller and more defected, resulting in better electrical conductivity. The improved property as evidenced by the larger Urbach energy is attributed to the higher efficiency of hydrogen reduction due to smaller grains. The higher lattice scattering observed in the behavior of the temperature-dependent resistivity is caused by the more defected grains.