A novel electrochemical sensor basing on an indium tin oxide (ITO) glass electrode modified by hydroxyapatite (HAp) film was fabricated by electrodeposited method for simultaneous detection of Pb2+, Cu2+ and Hg2+ using differential pulse voltammetry (DPV). Various experimental parameters such as supporting electrolyte and pH were optimized. Under the optimal conditions, the electrode needs no accumulate time. The electrode showed linear correlation in the ranges of 1 x 10(-7) to 1 x 10(-6) M and 3 X 10(-6) to 1 X 10(-5) M for Pb2+, 1 x 10(-7) to 1 x 10(-6) M and 1 x 10(-6) to 1 x 10(-5) M for Cu2+, and 1 x 10(-7) to 1 x 10(-5) M for Hg2+. The limits of detection (S/N = 3) were 1 x 10(-12),1 x 10(-12), 8 x 10(-8) M for Pd2+, Cu2+ and Hg2+, respectively. Meanwhile, the developed electrode showed excellent anti-interference capabilities and selectivity with other heavy metal ions and common ions. The electrode has been successfully applied to the detection of above heavy metal ions in tap water. Above all, two major detection mechanisms of developed electrode were also researched in this work. The mechanism of filling Ca deficiency due to that the radiuses of Pb2+ and Cu2+ are similar to Ca2+, so Pb2+ and Cu2+ can fill in the deficiency. That is why the detection limits of Pb2+ and Cu2+ are much lower than Hg2+. When Ca deficiency has been almost filled, the detection of Pb2+ and Cu2+ based on the mechanism of surface adsorption. Because the radius of Hg2+ is not similar to Ca2+, Hg2+ cannot fill in the deficiency. The detection of Hg2+ mainly based on the mechanism of surface adsorption. The OH- on the surface of HAp plays a major role in the adsorption of heavy metal ions. (C) 2018 Elsevier Ltd. All rights reserved.