Purpose Coronary Artery Disease (CAD) remains a major cause of morbidity and mortality in the world. Low vitamin D status has been shown to be associated with increased risk of developing cardiovascular disease, hypertension and obesity. We planned to research the association between low vitamin D status and the severity of CAD. Procedures A total of 348 consecutive patients undergoing coronary angiography for evaluation of CAD were included in this study. 25-Hydroxyvitamin D 25(OH)D was measured by chemiluminescence assay. CAD severity was assessed by using the SYNTAX scores. The data presented are the mean levels/values and standard deviation. Findings The serum 25(OH)D level of CAD patients was 18.2 ± 10.6 ng/ml. The SYNTAX scores were 27.8 ± 8.5. In a multivariate linear regression analysis (adjusted for age, high-sensitivity C-reactive protein, SYNTAX score, parathyroid hormone, body mass index, haemoglobin and creatinine), the serum 25(OH)D level showed a negative correlation with SYNTAX score and high-sensitivity C-reactive protein (hsCRP) level. Logistic regression analysis identified 25(OH)D as an independent factor related to high SYNTAX scores. Patients whose vitamin D levels were in the lowest 25(OH)D category (<20 ng/ml) were more often in the high SYNTAX scores group, with their incidence about two-fold higher than those in the highest 25(OH)D category (>30 ng/ml). Conclusion Low vitamin D is associated with the severity of coronary artery stenosis.