A honeycomb structural body (1) is obtained by bonding, into one piece, a plurality of honeycomb segments (2a, 2b) having a plurality of through-holes (6) surrounded by partition walls (10) and extending in the axial direction. An average wall thickness of at least one honeycomb segment (2a) not constituting the outermost peripheral surface (23) of the honeycomb structural body is larger than an average wall thickness of at least of each honeycomb segment (2b) constituting the outermost peripheral surface (23) of the honeycomb structural body. A honeycomb structural assembly is obtained by providing a material B having compressive elasticity on the outermost peripheral surface (23) of the honeycomb structural body (1) in a compressed state and thereby compression-holding the honeycomb structural body (1) in a metallic container. The honeycomb structural body and assembly have low reduction in the conversion rate, purification efficiency, regeneration efficiency, etc., during use, and are superior in durability against breakage caused by thermal stress.