Is using a treadmill a good way to lose weight?

Any activity that gets your heart rate up, and keeps it up, will make your body use up more calories.

If your eating is under control, some of those calories will be from stored fat, so you will lose weight.

If you're overeating, it's really hard to lose weight from exercise alone. Not many people can push hard enough, long enough to use up any important amounts of calories that way.