We demonstrate that the electron injection barrier (Δe) between Co and C60 can be tuned by inserting a thin Alq3 interlayer. Using ultraviolet photoemission spectroscopy, Δe of C60 on Alq3-predecorated Co (Δe=0.3 eV) was found to be reduced by 0.3 eV compared with that of C60 deposited directly on the bare Co metal surface (Δe=0.6 eV). Due to Fermi level pining at the Alq3/Co interface, this tuning effect is independent of the thickness of Alq3 interlayer from multilayer to monolayer. Based on the experimental results, band level alignment diagrams are proposed for C60/Co and C60/Alq3/Co interfaces with two different Alq3 thicknesses. Our findings could have potential applications for the reduction in the carrier injection barrier in organic spin valves.