Abstract: Objective To evaluate the effect of thermal cycling on surface microstructure of different light-curing composite
resins. Methods A nanofilled composite (Z350) and 4 microhybrid composites (P60, Z250, Spectrum, and AP-X) were
fabricated from lateral to center to form cubic specimens. The lateral surfaces were abrased and polished before water storage
and 40 000 thermal cycles (5/55 ºC). The mean surface roughness (Ra) were measured and compared before and after thermal
cycling, and the changes of microstructure were observed under scanning electron microscope (SEM). Results Significant
decreases of Ra were observed in the composites, especially in Spectrum (from 0.164±0.024 μm to 0.140±0.017 μm, P<0.001) and
Z250 (from 0.169±0.035 μm to 0.144±0.033 μm, P<0.001), whose Ra approximated that of P60 (0.121±0.028 μm) with smoothly
polished surface. SEM revealed scratches and shallower pits on the surface of all the 5 resins, and fissures occurred on Z350
following the thermal cycling. Conclusions Water storage and thermal cycling may produce polishing effect on composite
resins and cause fissures on nanofilled composite resins.