New amorphous alloys exhibiting a wide supercooled liquid region before crystallization were found to form by melt spinning in wide composition ranges of LaAlM, MgYM and ZrAlM (M = Ni or Cu) systems consisting of the constituent elements with significantly different atomic sizes. The temperature span between glass transition temperature, T g, and crystallization temperature, T x, ΔT x ( = T x − T g) is > 50 K in the compositional ranges around La 2AlM, Mg 6Ln 3M and Zr 3AlM and the largest ΔT x reaches 126 K. The critical cooling rate for the glass formation, R c, is as low as 87–115 K/s and T g/ T m is > 0.6 in the composition range where ΔT x > 50 K. There is a clear tendency for R c to decrease with an increase of ΔT x and T g/ T m. The crystallization of the alloys with large ΔT x occurs through the simultaneous precipitation of several compounds. Based on these results, it is presumed that the large glass-forming ability for these alloys is due to a combined effect of the difficulty of long-range atomic redistribution required for the precipitation of the compounds, the rapid increase of viscosity with decreasing temperature and the large liquidus-solidus interfacial energy which originates from the optimally bonding and packing states resulting from large negative heat of mixing and large atomic size ratios.