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Zr57Cu20Al10Ni8Ti5 has very high glass forming ability (GFA) due to its low crystal growth rate which limits development of crystallites arising from container walls or nuclei. Effect of ZrO2 and SiO2 on GFA was studied by investigating the solidification microstructures of alloys containing different amount of the oxides. Amorphous ingots were formed by melting and freezing alloys without addition of the oxides using an arc furnace. Partially amorphous ingots were formed for addition of the oxides up to 0.2 wt %. Amorphous forming conditions of alloys containing greater concentration of the oxides were studied using a drop tube. GFA decreases with increasing concentration of the oxides. However, superheating of melts eliminates some effects of the oxides. Different types of oxides leads to different microstructures due to their different heterogeneous nucleation rates and nucleation temperature.