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The thickness dependent spin-orbit torque (SOT) in an L1 0 -FePt single layer is investigated in this work. As the thickness increases from 8 nm to 16 nm, the magnetization switching ratio in the L1 0 -FePt film with higher chemical ordering becomes smaller. It is noted that compared with 3-nm-thick L1 0 -FePt film, 8-nm-thick L1 0 -FePt film can switch much magnetization with the increase of chemical ordering. When the FePt film is thick enough, the SOT in FePt is closely related to the L1 0 -ordered structure, which indicates a bulk nature. Therefore, the disordering plays an important role in the magnetization switching only for the ultra-thin FePt films, while the structural gradient may play an important role for thicker films. However, both of the two mechanisms cannot fully explain the process of magnetization switching and the spin current generation. Although many factors influence SOT, here in this work we emphasize only the bulk nature of strong SOC in L1 0 -FePt through density functional theory calculations, which should generate large spin current due to spin Hall effect.