Hydrogen is an essential impurity in diamond. The tendency of hydrogen concentration shows hydrogen in the bulk is intrinsic to natural and HPHT synthetic diamond, but not to CVD synthetic diamond. The result of modern measure technology (vibration spectroscopy) measurement has proved that the chemical states of hydrogen include C－H bond, H2O, hydrogen molecule, －OH and N－H bond in the bulk of diamond. In addition, The research of micro-inclusions of diamond implies HF and H+ will be other chemical states of hydrogen. The research of location which hydrogen stays in diamond structure has concluded that hydrogen is firmly fixed in diamond in the form of lattice site, bond-centered site, anti-bond centered site and tetrahedral interstitial site, also anchored in the micro-inclusions of diamond in the form of H2O and CH4 etc.The sequence of stabilization is C-H bond> anti-bond centered site> bond-centered site > tetrahedral site. The research of dynamic mechanism of hydrogen suggest that Annealing the diamond up to 1473K produces no evidence whatsoever of any movement of the implanted hydrogen, so we forced to conclude that the hydrogen is self-trapping in the bulk of diamond. The nuclear probes technology (especially ERDA) is l immediate and sensitive in the study of the hydrogen concentration in diamond and the vibration spectroscopy (especially FTIR) is effective in the study of chemical states of hydrogen. The research of hydrogen in diamond has promoted the development in the mantle's mineralogy, material sciences and gemology etc.