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The effect of crystal size and initial dislocation density on surface roughness evolution in FCC single crystals during the early number of cycles of mechanical cyclic loading is investigated using three dimensional discrete dislocation dynamics simulations. Crystals having size less than 2 μm show early development of surface slip localization, while larger ones show a more uniform distribution of surface steps. The surface roughness is found to increase with increasing number of loading cycles with larger crystals showing a high roughening rate compared to smaller crystals. Double cross-slip is observed to be the main mechanism that derives the development, growth and thickening of surface slip bands. The maximum surface height, which is an indicator of the surface stress concentration is observed to increase linearly with the number of loading cycles and quadratically with the crystal size for the simulated …