Objective To evaluate plasma Cystatin C (CysC) for early diagnosis of contrast induced nephropathy (CIN).Methods Firstly, 50 patients were enrolled.The vein blood was obtained before and 2, 4, 8, 24 and 48 hours after procedure for the plasma CysC measure used enzyme-linked immunosorbentassay kit to determine optional time-point for sampling.All the 311 patients were continuously enrolled in study with established or suspected coronary artery disease combined with mild or medium renal dysfunction which was predetermined with the estimated glomerular filtration rate (eGFR) of >30 ml·min-1·1.73m-2 and ≤90 ml·min-1· 1.73m-2 according to the formula of modified diet in renal disease.CIN was defined as a relative increase in serum creatinine (SCr) from baseline of ≥25% or an absolute increase of ≥0.5 mg/dL (44umol/L) up to day 3.All the patients were arranged to CIN and non-CIN groups according to whether developed CIN or nor.Results At baseline, the plasma CysC was associated with Scr with 0.340 of r (p<0.001).Plasma CysC increased at 2 hour and reached peak at 24 hour after contrast exposure, thus we determine optional time point at 24 hour after procedure for CysC measure.Compared with baseline, the CysC level at 24 hours after procedure increased significantly among CIN [(554.20±198.69)vs.(456.96±136.23)μg/L] as well as among non-CIN [(468.68±249.92) vs.(399.84±182.55)μg/L].The sensitivity and specificity of CysC was 76.9% and 81.2% According to cutpoint of relative increasing more than 25%.The negative predictive value was 96.1%.Conclusion Plasma CysC might be useful biomarker for early diagnosis of CIN with higher negative predictive value.