ITO thin films (100-200 nm) are deposited on glass and plastic (PET and polycarbonate) substrates by r.f. sputtering. Process parameters such as oxygen partial pressure, r.f, power, and post deposition annealing parameters are varied to determine the dependence of the sheet resistance on process parameters. The microstructure of these thin films is determined using an X-ray diffractometer (XRD) and a transmission electron microscope (TEM). The experimentally observed dependence of the sheet resistance on the grain size and grain orientation of these films is correlated to the dependence of the electron mobility on grain boundary scattering. Larger grain sizes (approximate to 25 nm) in ITO films result in lower sheet resistance (250 Ohm/square). This type of large grain size microstructure is produced with moderate r.f. power (approximate to 100 W) and low oxygen partial pressure (approximate to 10%). There is a unique correspondence between grain size and grain orientation. ITO films with a strong peak intensity ratio of (400) orientation to all other orientations (approximate to 0.35) have the largest grain size (approximate to 25 nm) resulting in the lowest sheet resistance (250 Ohm/square) and high transmission( approximate to 86.7%) at lambda = 550 nm.