Which fat burns on stepper machine?

There's no way to target fat burn, the body will draw a little from all over. But the body will use the easiest available fuel first, leaving the fat(which is harder to turn into energy) until last. Unless you go long enough you'll never see a fat change.

Also, there's basically no way to lose weight/fat through exercise only, you have to start with controlling your diet. One muffin is about the energy equivalent of one solid hour working your heart out, so you've got to control what you're eating to get anywhere.