Skyrmion is a topologically stable field configuration which was originally proposed to describe nucleons by Tony Skyrme in 1962. In condensed matter physics, it was noticed in theory that a skyrmion could actually exist as a topological twist of two-dimensional spin texture. It was only recent that magnetic skyrmions were finally discovered in several class of materials and have soon become an emerging field in nanomagnetism research. As an alternative approach to the naturally formed skyrmions, we fabricated single crystalline Co disks on perpendicularly magnetized Ni/Cu(001) film to create artificial skyrmions. In this way, we can tailor the skyrmion topology by changing the relative orientation between the vortex core polarity and the surrounding perpendicular magnetization. Using Photoemission Electron Microscopy (PEEM) to image the spin textures after applying an in-plane magnetic field, we demonstrated clearly the topological effect in the skyrmion core annihilation process.