80.7W-13.2Ni-6.1Fe(atom percent)tungsten heavy alloys were prepared by mechanical alloying (MA). The volume fraction of the remanent crystalline W in the as-milled materials and the W atom percent in amorphous phase were measured using a method similar to the internal standard method and the route of amorphization. The mechanism of the amorphous phase forming in the process of milling was analyzed. The results show that the crystalline size of tungsten reaches 11 nm~20 nm after milling for 60 h. Ni dissolves in crystalline W and then amorphization of W supersaturated solid solution occurs after milling for 20 h. Forming of the supersaturated solid solution is attributed to dissolving of the small particles with large stored energy into W continuously and the critical size of the Ni particles which can dissolve in the W is 3nm calculated based on the stored energy model. During milling, the volume fraction of the remanent crystalling W is decreased, and the W contents in the amorphous phase remain content, approximately 63W-37Ni(Fe), due to the Fe contamination dissolving into W continuously.