Synergic effects of rare earth-magnesium based hydrides (LaH3-MgH2) on thermal dehydrogenation of potassium borohydride (KBH4) are discussed in the paper. Comparing to above 675 K for the LaH3-MgH2 or KBH4, the dehydrogenation temperature can be decreased to 346 K for the composites (LaH3-MgH2-KBH4). The apparent activation energy of desorption process for the mixture is also greatly reduced. On the other hand, the products in the temperature programmed desorption process are also illustrated.