In this paper I investigate the underwriting activities of property-casualty insurers in United State from 2001 through 2015 to determine whether insurers herd in underwriting.I particularly examine whether insurance underwriting movements are due to adjustments for fundamental changes or over-adjustments resulted from herding.I find that fundamental changes cannot fully explain the movements of insurer underwriting and that insurers tend to follow their peers to increase or decrease underwriting.Insurers are also fund to be more responsive to the herd of supply cutting than the herd of supply expanding.Further more,herding imposed greater impact on large firms and small firms(compared to medium-size firms),mutual firms and young firms.Firms of larger size and older age are more likely to be part of the herd.