A three-Dimensional deflection of the electron-beam resulting a missed joint due to thermoelectric magnetism generated in welding dissimilar metal is experi- mentally and analytically investigated. In theoretical analyisis a narrow welding cavity is assumed to be a paraboloid of revolution. Applying a three-dimensional analytical solution of thermolectric currents, magnetic fiux densities, and deflections of the electron beam are determined from Maxwell's electromagnetic equations. The computed magnetic fields,thermoelectric currents and beam deflection will be compared to experimental results. Factors affect-ing deflection are discussed. The major advantage of welding with a high-power-density-electron-beam is the ability to weld dissimilar metals, unfortunately, can be missed. when the beam from the electron gun is properly aligned with the joint, subsequent deflection of the beam can result in nonsymmetric fusion along a joint, or the fusion zone may miss the joint .One reason responsible for the beam deflection is due to thermoelectric magnetic fields. Since temperature gradients exist between the top and bottom and in front and be-hind the deep and narrow cavity ,thermoelectric currents due to the Seebeck effect are produced in dissimilar metals. The induced magnetic field above and below the top surface therefore deflects the electron-beam and induces a missed joint in S-shaped. In this study, experiments will be conducted to measure missed joints. A three-dimensional thermoelectric and heat conduction model is also to predict deflection of an electron-beam From the surroundings and to bulk workpieces, the entire trajectory of the beam can be determined.a more systematical and realistic understanding on missed joint parameters.