Degenerative cervical spondylolisthesis (DCS) is a cervical deformity arising from regressive changes where trapezoidal deformity characterized by hypertrophic osteophytes of spinal elements is often observed. There is a paucity of literature about the spinal contour of DCS and trapezoidal vertebrae. We conducted this research to clarify the relationship between spinal sagittal alignment and trapezoidal deformity in DCS. Total seventy-nine patients with cervical spondylosis were enrolled. Twenty-four patients who exhibited cervical spondylolisthesis were classified into DCS group. Other patients were classified into a control group. Measurements of radiographic parameters and trapezoidal deformity were made. DCS was found mostly in C3-C4 and C4-C5 (16 and 10 cases, respectively). T1S and T1-T4 TK was larger in the DCS group than in the control (T1S: 29.9 ± 2.3° vs. 23.7 ± 1.5°, T1-T4 TK: 14.9 ± 2.1° vs. 9.0 ± 1.4°). C2-C7A was smaller in DCS (3.5 ± 3.6° vs. 11.9 ± 2.3°). Trapezoidal deformity was apparent in the vertebra below the slipped segment. Among sagittal parameters, T1S and T1-T4 TK were positively correlated with DCS (r = 0.523 and r = 0.438, respectively). For these correlations with DCS, both logistic and linear regression models predicted threshold values of approximately 30° for T1S and 15° for T1-T4 TK responsible for DCS. DCS was mostly found in the middle cervical region. Among sagittal parameters, enlarged T1S and T1-T4 TK, which were strongly correlated with amount of slippage, was considered affected to DCS. Cervical kyphosis and trapezoidal deformity also exhibited strong correlations with DCS, and were considered responsible for clinical instability.