Metal-organic chemical vapor deposition (MOCVD) has been used to deposit conductive oxide RuO2thin films at different temperatures on (100) MgO and (100) LaAlO3substrates. The microstructural properties of the RuO2films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Different growth and microstructural properties were observed for the films deposited on the two substrates. For example, the films on MgO are epitaxial at deposition temperatures as low as 350°C, and the films on LaAlO3are epitaxial only at deposition temperatures of 600°C and above. The epitaxial films on MgO consist of two variants with an orientation relationship given by (110) RuO2//(100) MgO and [001] RuO2//[011]MgO. The epitaxial films on LaAlO3, on the other hand, contain four variants with an orientation relationship given by (200)RuO2//(100)LaAlO3and [011] RuO2//[011] LaAlO3. The difference in growth and microstructural properties were explained based on geometrical considerations for the film and substrates. The RuO2/substrate interfaces and the RuO2domain boundaries were also investigated by TEM and HREM.