Electrochemical behaviors of milled Si using high energy mechanical milling (HEMM) process are characterized with a degree of amorphization. The amount of amorphous Si increases with milling time, and Si crystallites are embedded in the amorphous Si matrix. Unlike crystallite Si, partially amorphous Si starts to react with Li near 0.32 V. An intermediate LiSi phase is identified at 0.17 V during the first discharge of amorphous Si with Li. The milled Si/graphite (m-Si/G) composite prepared by using the simple HEMM process shows excellent electrochemical performance with a reversible capacity of 800 mAh g−1 at a rate of 0.5 A g−1 when cycled between 0.11 and 2.0 V.