Glomerular filtration rate ( GER) is measured either by exogenous or endogenous markers like serum creationine, which is offected by muscle mass, height and body composition. Cystain C and B2 microglobylin ( B2MG) are used to overcome these limitations Aim of work: Evaluation of cystatin C as an early marker of GFR in pediatric practice . Methods: The study was conducted on 60 children with renal impairment. Results : Reference value of cystation C is 0.56-1.1mg/l; independent on age and sex. There is strong correlationbetween GFR and reciprocal of both cystatin C and creatinine ( 0.948, 0.937) and negative correlation with B2MG –0.777, P<0.01, ROC analysis show the AUC for cysratin Cis higher that creatinine and B2MG .