The 2% (mass fraction of Al2O3) Al-doped ZnO (ZnO∶Al) thin films were sputtered on glass and polyimide (PI) substrates by radio-frequency (RF) magnetron sputtering technology. The effects of substrate materials on the structural, electrical and optical properties of ZnO∶Al thin films deposited on different substrates are studied. It is found that substrate materials have significant influence on film crystallization and resistivity but little on optical transmittance. Highly c-axis oriented ZnO∶Al films in (002) direction are observed on both glass and PI. Besides, it is manifested that the average optical transmittance in the visible-light range (400～800 nm) is around 85% for both films. Films on glass presents stronger (002) diffraction peaks and lower full-width at half maximum (FWHM). The lower resistivity of 2.352×10-4 Ω·cm is obtained in samples deposited on glass. Also, films on glass show larger grain size and denser microstructures than films on PI. Meanwhile, the ZnO∶Al films deposited on PI also own good crystallinity and low resistivity of 6.336×10-4 Ω·cm, which make them suitable as window materials in flexible solar cells. Films on glass are available as transparent electrodes in flat panel displays and solar cells.