In order to investigate the effect of environmental degradation factors (ozone and temperature) and strain on crack initiation of base-isolated natural rubber bearing for bridges, a series of accelerated ozone exposure experiment with different temperature were carried out. It was confirmed that the high temperature condition was severer than the low temperature one for ozone crack initiation of natural rubber. The crack was initiated around the region, in which the tensile strain was generated due to shear deformation of rubber bearing, under the high ozone density (150pphm) and the high temperature (40°C) condition. The mechanical performances of rubber bearing with cracks were not deteriorated because these cracks initiated in the surface rubber did not reach the inside rubber layers.