BACKGROUND The finite element analysis (FEA) was used to explore the effect of different graft heights on adjacent segment and graft segment stress after C4/5 anterior cervical discectomy and fusion (ACDF). MATERIAL AND METHODS A detailed, geometrically accurate 3-dimensional cervical spine model was successfully built from computed tomography (CT) scanning of a healthy adult male. We changed the graft height in C4-C5 to be 90%, 150%, 175%, and 200% of the preoperative disc height and simulated the postoperative scenarios with different bone graft height, respectively. A stress analysis was conducted on the adjacent segment and graft segment. RESULTS The maximum von Mises stress on C3-C4 showed that when the graft height was 200%, the values were 0.99 MPa, 0.85 MPa, 0.91 MPa, and 0.89 MPa in different loading conditions. For C5-C6, the maximum von Mises stress was 0.77 MPa, 0.83 MPa, 0.91 MPa, and 0.81 MPa, observed when the graft height was 175%, except in extension condition. With regard to graft segment (C4-C5), the biggest von Mises stress was 1.25 MPa, 1.77 MPa, 1.75 MPa, and 1.81 MPa observed at 200% graft height. For these 3 segments, the smallest von Mises stress was found at 150% graft height under the 4 loading conditions. CONCLUSIONS The graft height makes an important difference on the stress on the adjacent segment and the graft segment after anterior cervical discectomy and fusion. A 150% graft height was considered the proper graft height in C4/C5 ACDF, with the lowest stress on the adjacent segment and the graft segment.