Thoughts stopped when seen them. Either happy or sad feelings of mind disappears by seen them. Is this impermanence?
Yes. This is impermanence, at the experiential level. You are getting the hang of the fact that experiences come and go, and they get affected by mere looking at them.
There is also impermanence at a deeper or subtler level, where every sensation becomes ripply or vibratory - that can be seen when mind is trained to see impermanence at a deeper/subtler level.
edit: I don't know a better term to use than ripply to describe it, anyways my usage of the term comes from Shinzen Young's meditation manual, page 41. (A beautiful illustration of meditation path up to the arising and passing away stage can be found in pages 40-45)