A novel DNA electrochemical probe(locked nucleic acid，LNA） was designed and involved in constructing an electrochemical DNA biosensor for the detection of PML/RARα fusion gene in acute promyelocytic leukemia(APL). This biosensor was based on a “sandwich” detection strategy，which involved a pair of LNA probes，eg.hairpin capture probe and reporter probe. Streptavidin-HRP was bound to biotin labeled at the end of reporter probe via streptavidin-biotin affinity binding. In the presence of hydrogen peroxide(H2O2），HRP catalyzed the oxidation of the substrate 3,3′,5,5′ tetramethylbenzidene(TMB） to offer an enzymatically amplified electrochemical current signal for the detection of target DNA. This sensor was applied in the direct quantitative detection of synthetic PML/RARα fusion gene in PBS buffer. The results indicated that the biosensor showed an excellent specificity to distinguish the complementary sequence and different mismatch sequences. A linear relationship between the amperometric signal and the target concentration was obtained in the range of 1.0×10-11-1.6×10-10 mol/L with a detection limit of 1.0×10-13 mol/LIn addition，the biosensor was used for the determination of PML／RARα fusion gene in human serum samples without dilution with high sensitivity,selectivity and good repeatability.This method would be expected to use in real sample for further solving the actural problems of early diagnosis and prognosis monitoring of APL.