The purpose of this study is to investigate the effect of armature design on the thermal and magnetic induction distribution of the rails and the armature in an electromagnetic launcher. In our formulation of governing, non-linear differential equations, Maxwell equations coupled with energy equation are applied to the rails and the armatures. To solve the non-linear governing differential equations, a finite difference code based on alternative directional implicit method is utilized. For different armatures, the length, shape, and input current of the rails stay the same. In addition, armature-melting latent heat and the friction force between the armatures and the rails are considered. First, armature speed is calculated; then, temperature and magnetic induction distribution is calculated by the energy equation. Temperature and magnetic induction distribution obtained for the rails and different armatures show that the maximum temperature occurs at the trailing edge of each armature. The best design shows the lowest temperature, which is about 600 K. This is due to aerodynamic shape and stability of this armature. However, for all armatures, the temperature of one meter rail stays around 360 K.