Two-dimensional (2D) materials received large amount of studies because of the enormous potential in basic science and industrial applications.Monolayer Pd2Se3 is a fascinating 2D material that was predicted to possess excellent thermoelectric,electronic,transport,and optical properties.However,the fabrication of large-scale and high-quality monolayer Pd2Se3 is still challenging.Here,we report the synthesis of large-scale and high-quality monolayer Pd2Se3 on graphene-SiC (0001) by a two-step epitaxial growth.The atomic structure of Pd2Se3 was investigated by scanning tunneling microscope (STM) and confirmed by non-contact atomic force microscope (nc-AFM).Two subgroups of Se atoms have been identified by nc-AFM image in agreement with the theoretically predicted atomic structure.Scanning tunneling spectroscopy (STS) reveals a bandgap of 1.2 eV,suggesting that monolayer Pd2Se3 can be a candidate for photoelectronic applications.The atomic structure and defect levels of a single Se vacancy were also investigated.The spatial distribution of STS near the Se vacancy reveals a highly anisotropic electronic behavior.The two-step epitaxial synthesis and characterization of Pd2Se3 provide a promising platform for future investigations and applications.