Compared with the conventional magnetostriction in Ni alloys which are in the order of several tens ppm (Parts Per Million =10^-6), RFe₂(R = rare earth element) Laves Phase intermetallic compounds show large saturation magnetostriction in the range of a few thousands ppm. However, the large external magnetic field necessary to obtain saturatio magnetostriction has due to large magnetocrystalline anisotropy energy restrained the applicationof magnetostriction materials in RFe₂intermetallic compounds. As a result of its solution, the largest published value of effective giant magnetostriction in a low external magnetic field (less than a few hundred Oe) is reported in this paper by means of amorphisation of RFe₂ intermetallic compounds with the addition of boron, as a half metal. For the amorphous (SmFe₂)_0.97 B_0.03 alloys, the effective magnetostriction of -545 and -610×10^-6 is obtained at 400 and 1,000 Oe, respectively. Moreover, the effective magnetostriction of 590 and 630×10^-6 in the amorphous (TbFe₂)_0.98 B_0.02 alloys is also found at 400 and 1,000 Oe, respectively. This result will provide a clue to understanding the effect of half metal on anomalous increase of the effective giant magnetostriction and attract the great attention for magnetostriction applications.