Quark-Gluon Plasma (QGP) is a new state of matter expected by QCD in hot and dense state. The existence of QGP is expected in the early universe, inside of a neutron star, and relativistic high energy heavy ion collisions. The last one is only the tool we can study on the earth. In this seminar, I will present results of experimental study for QGP at the Relativistic Heavy Ion Collider (RHIC) at BNL. While there are some probes to search and study QGP, I will talk some key measurements with a focus on the investigation using lepton channels.