Diffusion bonding between Al and Cu was successfully performed by hot isostatic pressing (HIP). To improve the strength of diffusion bonding joint, pure nickel foils with different thickness were used as intermediate layer. Microstructure of the interface between Al and Cu was investigated by X-ray diffraction (XRD) technique, secondary electron microscopy (SEM), and nano-indentation tests. When the temperature was 500 ℃ and held for 3 h with a processing pressure of 50 MPa, Al and Cu could be bonded with its interface formed by several diffusion layers. With the addition of Ni interlayer, the diffusion of aluminum atoms was effectively hindered, and the interface became smoother. The tensile strength of bonded joints increases with increasing the thickness of Ni interlayer, which contributes to a reduction in the thickness of intermetallic compounds (IMCs) and well bonding quality of Al-Cu joints.