A magnetic disk cartridge, which holds therein a magnetic disk with recording density and has a liner composed of polyethylene terephthalate fibers, is provided which achieves a good dust-removal effect by the liner without flawing the magnetic disk and without increasing the rotary torque of the magnetic disk. A magnetic layer of the magnetic disk is formed such that the magnetic layer contains diamond particles which have an average particle size satisfying a relationship “b−0.05≦a≦b+0.1” at 1% to 10% by weight with respect to the ferromagnetic material, where “a” represents the average particle size of the diamond particles in units of μm and “b” represents a thickness of the magnetic layer in units of μm. The fibers of the liner are selected from the fibers whose fiber diameter varies in its length direction.