In this study, the cobalt phosphide was fabricated on the carbon paper by using electrodeposition method. The morphology and composition of cobalt phosphide was controlled by bath composition and deposition potential/time. The morphology and composition of prepared CoPx catalyst was characterized by field emission scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Furthermore, transmission electron microscopy was utilized to examine the crystal structure and surface composition of CoPx catalyst at the surface. All electrochemical measurements to explore the HER activity were conducted by using conventional three electrodes cell system with N2-purged 0.5 M H2SO4 electrolyte. The HER catalytic activity and electrochemical surface area of CoPx was measured by using cyclic voltammetry. Especially, the electrochemical surface area was estimated by double layer capacitance measurement varying the scan rates. As a result, it was revealed that the intrinsic HER activity of prepared CoPx catalyst has significant relationship with their morphology and composition. Furthermore, the developed catalyst was adopted as hydrogen-evolving electrode for proton exchange membrane water electrolyzer (PEMWE).