Amorphous 50SiO . 50SnO (mol %) powders were synthesized by mechanical milling treatment of starting materials SiO and SnO in a dry N-2 atmosphere at room temperature. The resultant powders were used in a rechargeable cell, Li/1 M LiPF6, ethylene carbonate+diethylene carbonate/50SiO . 50SnO. For the first discharge at a current density of 1.5 mA cm(-2), the high capacity over 800 mAh g(-1) was obtained for the potential range of 0 to 2.0 V vs. Li/Li+. This first discharge capacity of amorphous 50SiO . 50SnO powders was much larger than that of amorphous materials in the system SnO-B2O3-P2O5 obtained by mechanical milling reported previously. In addition, irreversible capacity of the former was smaller than that of the latter. The amorphous 50SiO . 50SnO powders synthesized by mechanical milling treatments are one of the promising candidates as anode materials for lithium-ion secondary batteries.