The pi...pi stacking interactions between homogeneous dimers of C6FxI(6-x) (x = 0, 1, 2, 3, 4, and 5) have been investigated in detail using the state-of-the-art quantum chemistry methods. Computations dearly show that the pi...pi stacking interaction between the homogeneous dimer of C6FxI(6-x) is of the dispersion type interaction. At the same time, it is interesting to find that, for the pi...pi stacking interactions between homogeneous dimers of C6FxI(6-x), the M05-2X/def2-TZVPP computations give almost the same results as the CCSD(T)/SDD** computations. In the crystal growth and design, the formation of the pi...pi stacking interactions between homogeneous dimers of C6FxI(6-x) is always accompanied by the formation of the halogen bonds. Hence, competition and cooperation of the pi...pi stacking interaction and the halogen bond have also been studied theoretically by using C6FxI(6-x) and pyridine as coformers. At the M05-2X/def2-TZVPP theory level, it is found that the pi...pi stacking interactions in C6FsI center dot center dot center dot C6F5I and C6F4I2 center dot center dot center dot C6F4I2 are weaker than the corresponding halogen bonds in C6F5I center dot center dot center dot NC5H5 and C6F4I2 center dot center dot center dot NC5H5, and the pi...pi stacking interactions in C6FI5 center dot center dot center dot C6FI5 and C6I6 center dot center dot center dot C6I6 are stronger than the corresponding halogen bonds in C6FI5 center dot center dot center dot NC5H5 and C6I6 center dot center dot center dot NC5H5, while the strengths of the pi...pi stacking interactions in C6F3I3 center dot center dot center dot C6F3I3 and C6F2I4 center dot center dot center dot C6F2I4 are similar to the corresponding halogen bonds in C6F3I3 center dot center dot center dot NC5H5 and C6F2I4 center dot center dot center dot NC5H5. However, when the pi...pi stacking interaction and the halogen bond coexist, we find that the formation of the halogen bond will lead to the pi...pi stacking interaction much stronger, and vice versa.