The developments and applications about synthetic diamond and cubic boron nitrogen (cBN) were narrated in the paper. Moreover, their synthetic mechanism and character were compared. Being superhard materials, they are applied extensively in various industrial fields because of their good properties. They are considered as important materials to be developed in the developed countries because the purpose of improved economy benefits, energy saving, efficient and precision can be achieved by applying these materials. Further more, the materials are regarded as a sign of technology progress in a country.Impurities and heat resistance of synthetic diamond by belt-type press in the paper are studied, as follows.Through high precision way of neutron activation, the impurities in the diamond were analyzed. It is found that there are a few impurities, including most accelerant elements. In addition, many rare metal elements are also found, such as La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, As and so on. Meanwhile, the diamonds were heated fully in the MUFFLE stove. Then residual substances were analyzed in virtue of SEM and EDS. Various kinds of impurities were found in the inclusions, such as Fe, Ni, Ca, Al, Si, S, Cl, K, Na, Ti, Mg. As a result metal inclusions exist mostly in the diamond, especially the catalyst Fe-Ni alloy, while other impurities results from pressure mediums or carbon according to the above analysis ways.