Background: red cell distribution width is a marker associated with increased mortality and morbidity in cardiac patients, however it's relation with coronary artery calcium score (CACS) is not well studied yet. Aim of the work:  this study aimed to assess the relation between red cell distribution width (RDW) and coronary artery calcium score (CACS) in the diabetic patients undergoing coronary CT angiography. Patients and methods: this study was conducted on 60 patients presented for assessment of coronary artery disease (CAD) by coronary CT angiography and they were categorized into 2 groups, group (A) diabetics(30 patients),group(B)non-diabetics (30 patients), All patients included in this study were subjected to: History Taking, complete physical examination, multi-slice CT coronary angiography (MSCT) including calcium score(CACS), laboratory investigations including complete blood count (CBC) including RDW(SD&CV), serum calcium level(total and ionized), erythrocyte sedimentation rate (ESR), lipid profile. Results: higher RDW(SD) was associated with the presence of greater coronary complexity of CAD and higher calcium score. In our study total serum calcium and RDW (SD) were found to be independent predictors of high Ca score more than 100, while ionized calcium and systolic blood pressure(SBP) were independent predictors of high Calcium score more than 400. Cut off value of RDW to predict high calcium score (more than 100) was RDW(CV) more than 13.45 & RDW(SD) more than44.45, while Cut off value of RDW to predict high calcium score (more than 400) was RDW(SD) more than 45.1. Conclusions: a greater baseline RDW(SD) value was independently associated with the presence of a greater coronary complexity of CAD and higher calcium score.