The vortex phase diagram of high- T c superconductor YBa 2Cu 3O y (YBCO) single crystals has been studied by resistivity and magnetization measurements in magnetic fields up to 30 T . The vortex matter in YBCO shows a complex and rich phase diagram being affected by small change of oxygen content. Furthermore, the low-temperature scanning tunneling microscopy of slightly overdoped YBCO crystals in magnetic fields up to 5 T has been performed. We find that the vortex image at 4.2 K and 1 T forms a distorted triangular pattern with a hexagonal symmetry, but at 5 T it shows a disordered pattern indicating the vortex glass phase. These results are consistent with the vortex phase diagram resulted from macroscopic properties.