The　multi-fractal　de-trended　fluctuation　analysis　was　used　to　estimate　the　mental　stress　in　the　present　study.　In　order　to　obtain　the　optimal　fractal　order　of　the　multi-fractal　de-trended　fluctuation　analysis,　we　analyzed　the　relationship　between　singular　index　and　Hurst　index　with　order.　We　recorded　the　electroencephalogram　(EEG)　of　14　students,　compared　the　relationship　between　singular　index,　Hurst　index　and　quality　index,　ensured　the　optimal　order　being　[-5,　5]　and　achieved　the　estimation　of　mental　stress　with　the　β　wave　in　the　EEGs.　The　result　indicated　that　Hurst　index　and　quality　index　of　the　EEGs　under　mental　stress　were　greater　than　those　of　EEGs　in　the　relaxing　state.　The　Hurst　index　was　gradually　decreasing　with　the　order　increasing　and　was　finally　approaching　a　constant,　while　the　quality　index　was　amplified　and　variation　of　amplitude　of　the　singular　index　was　more　obvious.　We　also　compared　the　amplitude　and　the　width　of　singular　spectrum　of　the　EEGs　under　the　two　conditions,　and　results　indicated　that　the　characteristics　of　multi-fractal　spectrum　of　the　EEGs　under　different　conditions　were　different,　namely　the　width　of　singular　spectrum　of　the　EEGs　under　mental　stress　was　greater　than　that　under　relax　condition.　Copyright　©　2017　by　Editorial　Office　of　Journal　of　Biomedical　Engineering.