This paper presents a highly sensitive micro sensor for trace determination of lead. The sensor consists of three microelectrodes including a working electrode, a counter electrode, and a reference electrode. On the working electrode surface of the sensor, a bismuth film was evaporated on the working electrode to enhance the sensor's sensitivity and a negatively charged functional film of Nafion for cation-exchanging was deposited to reduce the interference caused by anionic substances in the solution. Compared with traditional glassy carbon electrode based heavy metal sensors, the microfabrication process endows the proposed sensor with great potential in portable heavy metal analyzer industry. Determination of Pb (II) was performed in an acetate buffer solution (0.1 M, pH 4.5) using anodic stripping voltammetry (ASV). Low limit of detection (LOD) of 2.79 ppb is obtained when detecting trace lead with the deposition time of 300 s. This simple, but highly sensitive electrochemical sensor may lead to wide applications of portable monitoring trace metal ions.