Polymer Electrolyte Membrane Fuel Cell (PEMFC) is regarded as a potential alternative clean energy source for automobile applications. Key challenges to the acceptance of PEMFC for automobiles are the cost reduction, improvement in power density for its compactness, and cold-start capability. High current density operation is a promising solution for them. However, high current density operation under normal and sub-zero temperature requires more oxygen flux for the electrochemical reaction in the catalyst layer, and it causes more heat and water flux, resulting in the significant voltage losses. So, the theoretical investigation is very helpful for the fundamental understanding of complex transport phenomena in high current density operation under normal and sub-zero temperature. In this study, the numerical model was established to elucidate the impacts of mass transport phenomena on the cell performance through the numerical validation with experimental and visualization results. The results indicated that the higher current density operation causes non-uniform reaction distribution, resulting in lower cell performance under normal temperature and less accumulated produced water under sub-zero temperature. They also quantitatively indicated the limiting factors for this non-uniform reaction distribution such as heat and water transport in rib/channel direction, oxygen transport resistance near Pt area, and the water transport through the membrane.