Present paper deals with the structural, magnetic and transport studies of as-deposited as well as annealed Co/GaAs(001) thin film at different temperatures. The X-ray diffraction measurements show oriented growth of as-deposited Co film in the hcp (002) direction. However, the sample annealed at higher temperatures shows formation of ternary Co2GaAs phase at the interface. Corresponding magnetic and transport measurements show decrement in magnetization and resistivity with annealing temperatures. The observed reductions in magnetization and resistivity values are mainly attributed to the formation of ternary Co2GaAs phase at the interface. (c) 2005 Elsevier B.V. All rights reserved.