We report the synthesis of cubic spinel ZnCo 2 O 4 thin films and the tunability of the conduction type by control of the oxygen partial pressure ratio. Zinc cobalt oxide films were grown on SiO 2 (200 nm)/Si substrates by reactive magnetron sputtering method using Zn and Co metal targets in a mixed Ar/ O 2 atmosphere. We found from X-ray diffraction measurements that the crystal structure of the zinc cobalt oxide films grown under an oxygen-rich condition (the O 2 /Ar partial pressure ratio of 9/1) changes from wurtzite-type Zn 1?x Co X O to spinel-type ZnCo 2 O 4 with the increase of the Co/Zn sputtering ratio, D co D zn . We noted that the above structural change accompanied by the variation of the majority electrical conduction type from n-type (electrons) to p-type (holes). For a fixed D co D zn / of 2.0 yielding homogeneous spinel-type 2 O ZnCo 4 films, the type of the majority carriers also varied, depending on the O 2 /Ar partial pressure ratio: p-type for an O 2 -rich and n-type for an Ar-rich atmosphere. The maximum electron and hole concentrations for the Zn Co 2 O 4 films were found to be 1.37 × 10 20 c m ?3 and 2.41 × 10 20 c m ?3 , respectively, with a mobility of about 0.2 cm 2 /Vs and a high conductivity of about 1.8 Ω/ cm ?1 /.