A negative active material for a rechargeable lithium battery including a flake powder including a plurality of flakes, each flake including a plurality of silicon atoms and a plurality of oxygen atoms, wherein an oxygen atom amount for each flake ranges from 5 wt % to 36 wt % based on a total amount of silicon atoms and oxygen atoms, each flake having a thickness ranging from 30 nm to 500 nm and a ratio of an average longest dimension to the thickness ranging from 10 to 100.