In the iron-based superconductor Fe(Te,Se), a topological phase transition is expected by changing Te/Se compositions, and it is considered a promising route to manipulate the topologically superconducting states and Majorana zero modes. In this talk, I will report our recent work on the electronic structures of FeTe_xSe_1−x single crystals over a wide Te/Se composition range using high-resolution angle-resolved photoemission spectroscopy. We explicitly identify the topological phase transition from the band structure. The high-resolution measurements reveal that a special Te/Se composition exists, at which the Dirac point is located exactly at the Fermi level. Most importantly, we directly resolve Majorana zero modes from scanning tunneling spectroscopy measurements, and confirm that Dirac point at Fermi level is essential for realizing clean and isolated Majorana zero modes in vortex cores.