Are Treadmill better the outdoor walking?

Well, if all you've got outside are very hard surfaces, then a treadmill can offer some cushioning, which can be beneficial if you have feet/joint issues.

A treadmill also get you out of traffic.

But for fitness purposes, a walk outside is likely to be better. It'll be more varied, which is always a good thing. It'll also be less boring, making it more probable that you'll stick to your routine.