Using in-situ synchrotron X-ray diffraction, we have studied the epitaxial properties of Cu clusters electrochemically deposited on n-GaAs(001) substrates. The Cu clusters have (001) base planes and their  directions are aligned with the  directions of the GaAs(001) surface unit cell, but with a large angular spread of 4.5 degrees. Moreover, the Cu(001) planes are tilted with respect to the GaAs(001) substrate. The tilt of about 6 degrees did not show a preference for any substrate azimuth, giving rise to a ring-shaped (111)(Cu) reflection. This epitaxial tilt seems to be a novel mechanism of strain relief in an epitaxial system with a large mismatch of lattice constants and a large surface roughness.