The electrochemical reactivity of ball-milled MoO3 powders was investigated in Li rechargeable cells. High-energy ball-milling converts highly-crystalline MoO3 bulk powders into partially reduced low-crystalline MoO3−y materials with a reduced particle size. Both bulk and ball-milled MoO3 exhibit a first discharge capacity beyond 1100mAhg−1 when tested in the 0–3V (vs. Li/Li+) range, which is indicative of a complete conversion reaction. It is found that partial reduction caused by ball-milling results in a reduction in the conversion reaction. Additionally, incomplete re-oxidation during subsequent charge results in the formation of MoO2 instead of MoO3, which in turn affects the reactivity in subsequent cycles. As compared to bulk MoO3, ball-milled MoO3−y showed significantly enhanced cycle performance (bulk: 27.6% charge capacity retention at the 10th cycle vs. ball-milled for 8h: 64.4% at the 35th cycle), which can be attributed to the nano-texture wherein nanometer-sized particles aggregate to form secondary ones.