Objectives: The present study was performed to compare between serum creatinine and cystatin C in detecting GFR in relation to eGFR using radionuclide study.Methods: Thirty one adult type 2 uncontrolled diabetic patients of a mean age of 53 years and disease duration more than 5 years, were subjected to history taking, clinical examination and laboratory investigation including serum creatinine, cystatin C and renal isotopes Tc-DPTA, with twenty healthy adults of matching age and sex served as a control group.Results: This study proved that serum cystatin C is highly significant and increasing with deterioration of kidney function. In addition, it found that cystatin C is more superior and increased in early nephropathy compared to serum creatinine when we compared microalbuminuria, macroalbuminuria and control. Therefore, this study suggests that cystatin C is more useful in detecting early decline in GFR in type 2 diabetes with microalbuminuria, and macroalbuminuria but not in renal impairment, as serum creatinine is more superior in renal impairment.Conclusion: we conclude that decline of GFR in macroalbuminuria in spite of normal serum creatinine. Cystatin C was more superior in estimating GFR in early diabetic nephropathy.