The most common permanent magnets assemblies that magnet housed with round steel shell are pot (cup) magnets, they are results of the development of magnets application, and widely used in magnetic holding,magnetic mounting. All types permanent magnetic material can be used to make pot (cup) magnets. Besides of protecting the inner magnets, the steel cups can transfer the magnetic line and make it into a magnetic circle. By this, the steel cup provides the magnets with more magnetic strength. Then the magnetic strength of whole magnet assembly is much more than equivalent round magnets. Rare earth neodymium is the best material to make the strongest pot(holding,mounting) magnets.