Hydrogen is a resource that provides energy and forms water only after reacting with oxygen. Among the many hydrogen generation systems, solid oxide electrolysis cells (SOECs) have attracted considerable attention as advanced water electrolysis systems because of their high energy conversion efficiency and low use of electrical energy. To find the relationship between operating conditions and the performance of SOECs, research has been conducted both experimentally, using actual SOECs, and numerically, using computational fluid dynamics (CFD). In this investigation, we developed a 3D simulation model to analyze the relationship between the operating conditions and the overall behavior of SOECs due to different contributions to the overpotential. Simulations were performed with various inlet gas compositions of cathode and anode, cathode thickness, and electrode porosity to identify the main parameters related to performance.