Objective To evaluate clinical significance of serum cystatin C (Cys C) in diagnosis of early renal impairment. Methods The serum Cys C levels of 136 patients with renal diseases were determined by immunoturbidimetry, and serum creatinine (SCr) were detected by Jaffe method. The glomerular filtration rate (GFR) measured by the method of endogenous creatinine clearance rate (Ccr) was used as golden standard, the correlation between GFR and the above two parameters were compared. The sensitivity，specificity and the reliability of Cys C were evaluated. Results The significant correlation was found between GFR, Cys C and Scr, and that the correlation between GFR and Cys C was the most strongest (P<0.01). There was significant statistical difference between Cys C and SCr (P<0.05). Conclusion Serum Cys C may be an accurate and reliable parameter to reflect the GFR in patients with impaired renal function and it might have guidance significance in diagnosis of early renal impairment.