Abstract: In this paper,capture DNAprobe with mercaptogroup was immobilized on a surface of gold electrode via selfassembly.Then,cocaine aptamer biosensor was constructed by immobilization of cocaine aptamer on the electrode through hybridization.Using tris (2.2' -bippridyl)dichlororuthenium (Ⅱ) hexahydrate as electroactive indicator,the quantitative determination of cocaine was carried out,based on the decrease of electrical signal of tris(2.2' -bippridyl) dichlororuthenium (Ⅱ) hexahydrate after the combination of aptamer with target cocaine.The effects of scan rates pH,the immobilizing time of capture probe and aptamer on the signal were investigated.The results showed that the biosensor exhibits linear response to cocaine in the range of 2 × 10-4—1.0 × 10-3 mol/L with a detection limit of 6 × 10-5 mol/L.The biosensor has some advantages,such as high stability and specificity.This sensor has been used to determine cocaine in a real serum sample.