We can find from the structure that the magnetism of permanent magnet is not always in power. Magnet is composed of magnetic domains. The so-called magnetic domains are small “paddy field” with magnetic moments. When these small “paddy field " is arranging in a certain regular pattern, the magnetic moments are not offset each other, then the magnet show magnetism outwards. While if the service time of the magnet is too long or if the magnets be heated or struck, the small "paddy field" will arrange in a chaotic way, thus, the magnetism of permanent magnet is diminish or disappear. If store neodymium magnets, alnico magnets, smco magnets, ferrite magnets under dry environment with temperature about 20°C, the magnetism can last for a long time.