Improved ultrafine sheath-core composite fibers and composite sheets composed of these fibers are disclosed. The composite fibers have a fineness in the range of 0.0001 to 0.5 denier and a core/sheath weight ratio in the range of 10/90 to 70/30. The core has a very high intrinsic viscosity within a specific range and is located at the center of the sheath. The sheath is composed of a polyester copolymerized with 1.5 to 8 mole % of 5-sodium (or lithium or potassium) sulfoisophthalate, based on the total acid component, of 5-sodium (or lithium or potassium) sulfoisophthalate and having a thickness of 0.04 to 2 microns. The composite fibers have a high strength and are dyeable with a cationic dye. When the composite fibers are combined with an elastic material such as polyurethane, suede-like artificial leathers having excellent softness, touch, feel and color can be obtained.