Transparent conducting In and Mg codoped ZnO (IMZO) thin films were deposited on quartz substrates at room temperature by radio frequency magnetron sputtering. The influence of Mg concentration (from 0 to 6 at.%) on the structural, optical and electrical properties of IMZO thin films was investigated in detail. All IMZO thin films have typical hexagonal wurtzite structure with a preferential orientation along the c-axis and show high average optical transmittance above 85% in the visible region. And, the optical band gap is tunable with the addition of the Mg dopants. The presence of indium favors energetically the formation of abundant zinc interstitial (Zn,) related shallow donor defects, thus greatly increasing the carrier concentration in the film. However, it was found that the electrical conductivity decreased as the Mg content increased. The main origin is related to the density reduction of Zn ; -related defects. Furthermore, the electrical properties of IMZO thin films can be significantly improved by thermal diffusion of extrinsic Zn atoms which creates extra Zn ; -related defects that provide electron carriers. These efforts will facilitate the development of ZnO-based TCO thin films.