The formation of surface relief on copper after exposure to nanosecond laser pulses at sub-threshold intensity in the condensed state has been experimentally studied. A system of micro-protrusions/depressions having a deformation character is formed in the irradiated zones near grain boundaries. Molecular dynamics simulation showed that the anisotropy of thermal expansion of differently oriented grains during cyclic heating to pre-melting temperature is the main reason for relief development, as thermal stresses arising in the subsurface layer exceed the yield strength of the material. Accumulation of defects with increasing energy density and number of pulses has been registered. The results are important for understanding the mechanisms of degradation of metal optics under thermocyclic pulsed loading. Keywords: polished copper, nanosecond UV laser, optoplastic effect, grain boundaries, plastic deformation, molecular dynamics simulation.