The purpose of this study was to clarify the relationships between strength characteristics, basic density and decay-area ratio of Japanese larch heartwood decayed by white-pocket rot fungus Polodaedalea chrysoloma. Sound trees and decayed trees of Japanese larch were logged from 117-yr old plantation forest, and green-wood specimens were collected from heartwood at 4.3 to 6.3 m above ground. Basic density, compressive strength, bending strength, bending Young’s modulus, dynamic Young’s modulus, shear strength, tensile strength, and decay-area ratio of the cross section were measured, and their relationships were investigated, respectively. Each strength and Young’s modulus of both sound and decayed specimens decreased as basic density decreased. It was suggested that basic density and each strength decreased as decay-area ratio increased. Basic density and strength properties could be estimated roughly by decay-area ratio of the cross section.