Grand unified theoriesu that attempt to unify electroweak and strong interactions via a single gauge principle contain two, enormously different mass scales: One scale Is dictated by the electroweak synthesis and believed to be around 102Ge VI c2 and another scale must exceed 1014Ge VI c2 in order to suppress exotic processes such as nucleon decay. In this class of theories it is crucial to ask how light (;Sl02GeV lc2 ) particles at low energies are affected by heavy (;;;:; 1014Ge VI c2) particles. Ordinarily, one expects2l that heavy particles decouple from the light sector and that effects of heavy particles to leading orders appear only through renormalization constants in an effective low energy theory, which is obtained by omitting all heavy particles. Validity of this decoupling theorem has been questioned 111 spontaneously broken gauge theories and a few examples against the decoupling have been found. 3 l In grand unified theories there is a special si tuation :4l An effective gauge synnnetry exists when one disregards the light mass scale m compared to the heavy mass scale lv!. It is therefore reasonable to expect from the very consistency of gauge theories that if the limit m->0 exists, there should be no correction of order Af/m to the light sector. Indeed, a proof of the decoupling has been constructed) at the oneloop level for a few special cases. A problem still facing us is that we lack a complete demonstration of the decoupling to arbitrary orders of perturbation. The purpose of this note is to extend our previous analysis 5 l to the three-point vertex and combine this result with the previous ones for the polarization tensor so that we can discuss low energy parameters in grand unified theories. We shall compute ratios of low energy coupling constants and ratios of light gauge boson masses, both of which are observables independent of renormalization schemes and can readily be compared with the results obtained by renormalization group arguments0l at low energies. These two results are found to agree if a correct identification of parameters is made. We first compute the renormalization constant Z1i for the proper vertex of a light gauge boson ~Vi· vVe take an arbitrary simple group G for the grand unification and denote the low energy gauge group by G0 consisting of a product of Abelian and non-Abelian subgroups, G/s. G0 Is presumably SU(2) )( U(l)