Abstract. We have implemented a fully implicit numerical approach based on space-time finite element methods for the Klein-Gordon equation in the 1(space)+1(time) dimension. The purpose of this paper is to present a stable and parallelizable numerical method. The proposed numerical method is applied to generate successful simulation results of spin-0 particle propagation in a charged scalar field. The time additive Schwarz method is vital to make successful simulations with KSP (Krylov Subspace Methods) solvers. The time parallelizable algorithm is implemented through PETSc(Portable, Extensible, Toolkit for Scientific Computation, developed by Argonne National Laboratory).1. Introduction. The Klein-Gordon(KG) equation is a partial differential equation(PDE) that governs the quantum evolution of wave functions for relativistic spinless particles. The KG equation describes a wide variety of physical phenomena. In paper [1], the KG equation includes classical wave systems such as the displacement of a string attached to an elastic bed and quantum systems based on scalar fields. The KG equation is also related to the Dirac equation. Modeling light matter interaction with the relativistic effect can be explained by the Dirac equation, and the KG equation can be applied to the relativistic effect.For another example, the electron-positron pair creation process in the supercritical breakdown of the fermionic vacuum is a striking prediction of the Dirac equation in [2]. It turns out that a quantum field theory based on the KG equation can predict similar phenomena from the Dirac equation in [3]. Therefore, the KG equation can provide new physical explanations about the universe, and can also provide clues to the Dirac equation because any solution to the Dirac equation is automatically a solution to the KG equation.In addition, the KG equation has many applications. The KG equation can be modified to a non-homogeneous model and a nonlinear model. A modified version of the KG equation can be used for many engineering models to explain solitary waves, and wave propagations. Paper [4] shows the Klein-Gordon system transmitting monochromatic waves. Paper [5] shows an Einstein-Klein-Gordon system (a massive scalar field coupled to gravity) responding to a numerical simulation of oscillation.Solutions of the KG equation both homogeneous and non-homogeneous are generally unavailable in [6]. Therefore, numerical simulation is required to estimate solutions. Papers [7] introduces numercial methods to the KG equation and KleinGordon-Schrödinger(KGS) equation without a forcing term. Paper [8] shows numerical approaches to the KGS equation without a forcing term.In this paper, we present to generate successful simulation results of spin-0 particle propagation in a charged scalar field. This problem is defined as a KG equation with forcing term. To efficiently solve the KG equation using numerical methods, we use various numerical procedures. First of all, we consider numerical stability, which is vital to simulate time dependent p...