Abstract In view of the promising role of the MOCVD technique in the advance of YBCO high- T c superconducting thin films grown for device applications, a simplified chemical vapor deposition system with a single mixed organometallic precursor was built and tested. Single phase YBa 2Cu 3O 7-δ films with T c values around 90 K were readily obtained on (100) YSZ and (100) MgO substrates with a normal precursor mass transport. Investigations indicate that, in addition to the gas phase stoichiometry as a major factor, the phase composition and crystallite orientations of the YBCO films were considerably affected by the oxidation agent (partial pressure and concentration) as well as the substrate temperature. The films could be either highly oriented (00 l) or (100) and / or (110) orientation dominated. A single phase Y 2Cu 2O 5 film with (002) orientation was obtained from a Ba deficient gas phase. The variation in superconducting behavior of the YBCO thin films deposited under different conditions was not only related to these phases but also to the growth orientation. The film uniformity and flatness are examined and discussed in terms of the gas flow pattern in the reactor.