Background: Accurate measurement of GFR is critical for the evaluation of new therapies and the care of renal transplant recipients. Although not accurate in renal transplantation, GFR is often estimated using creatinine-based equations. Cystatin C is a marker of GFR that seems to be more accurate than creatinine. Equations to predict GFR based on the serum cystatin C concentration have been developed, but their accuracy in transplantation is unknown.Aim of work: To assess the use of cystatin C as marker of renal function in renal transplant recipients.Objectives: the present study was performed to compare between S.cre and cyst. C in different methods of estimating the GFR by radionuclide study in detecting accurate renal function.Methods: 30 RTx patients(1 year ago) , 30 patients with CKD not on regular heamodialysis were subjected to history , clinical examination and laboratory investigation including S. cre., cr.CL., cyst C and renal isotopes Tc-DPTA& 30 healthy adults old age ˃60 years old.Results: this study proved that S.cyst. C is highly significant and increasing with deterioration of kidney function in all groups. In addition it is found that cyst C is more superior than S.cre. And increased in early nephropathy compared to serum creatinine and as accurate as renal isoptopes. Conclusion: we conclude that the measurement of cystatin C is useful in detecting early decline in GFR in renal transplant recepients.