Publisher Summary This chapter analyzes the problem associated with high aspect ratios when working on parallel incompressible computational fluid dynamics (CFD) algorithms. It considers a parallel semi-implicit method for pressure linked equations (SIMPLE) based incompressible axisymmetric CFD code. It is found that the grid aspect ratio has a strong influence on the convergence rate of the code on several processors relative to that on one processor. This phenomenon is analyzed and is attributed to the strong influence of the cell aspect ratios to the structure of the pressure correction equation—a Poisson-like equation. The degradation in convergence comes only when the domain is decomposed along a direction where the cell aspect ratios are high, and is attributed to the strong coupling in the pressure correction equations among sub-domains. This in turn affects the effectiveness of the parallel pre-conditioner for the conjugate gradient algorithm used in solving the pressure correction equation. The problem is further confirmed by studying the Poisson equation. The chapter suggests and demonstrates a remedy to improve the convergence of the parallel CFD algorithm.