The discovery of luminous quasars at z > 6 indicates the existence billion-solar-mass black holes at the end of reionization epoch. directly probing the early growth of supermassive black holes in the universe and the relation between the formation of early galaxies and black holes. Absorption spectra of these quasars trace the evolution of intergalactic medium (IGM). Currently, more than 60 quasars have been discovered at z > 5.5, including about 30 at z > 6. In this proceeding, I review the recent observational results studies of the highest redshift quasars, including the evolution of quasar density and luminosity function, the evolution of their spectral properties and chemical enrichment history and the evolution of AGN structure, and the evolution of Gunn-Peterson optical depth to probe the end of reionization epoch.