A pixel arrangement structure, an organic electroluminescent device (i.e., organic light-emitting diode device), a display device and a mask are provided, which can increase opening areas of a metal mask for forming the pixel arrangement structure, and improve the aperture ratio, the brightness, the service life and the image resolution of active matrix organic light-emitting diode (AMOLED) products. The pixel arrangement structure includes a plurality of pixels. Each pixel includes three sub-pixels of different colors; connecting lines of center points of the sub-pixels form equilateral triangles; and each sub-pixel has a shape of polygon with more than four sides.