Spinal fusion and total disc arthroplasty are used in the surgical treatment of cervical disc degeneration. When mobility is desired in the functional spinal unit, total disc arthroplasty is preferred instead of fusion. In this research, the effects of artificial disc prosthesis on the biomechanical behaviour of the C5-C6 functional spinal unit were investigated via finite element method. Firstly, three-dimensional CAD models of C5 and C6 vertebrae were created by using a computerized tomography images of a healthy human neck via 3D Slicer computer software. The mobility of the healthy model consisting of intervertebral disc, joints, and soft tissues was validated by previously published experimental studies in the field. On the second step, a ball and socket type artificial disc was defined using connector elements between C5-6 functional spinal unit to simulate the total disc arthroplasty method. Finally, to investigate the effect of prosthesis core radius changes on the biomechanical properties of functional spinal unit, healthy and implanted models were analysed via finite element method.