Mark seam breaks other uv island I would like if someone had same problem.
after editing Uv and made some separation, I found I need another seam to make the island to be straighten. 

But if I mark new seam, It breaks other UV island which I arleady made...
I hope to know how I can avoid this problem. thank you.
I guess what you need to do is pin your UV map:


Let say you've made your first unwrap with a seam. Select all in the UV Editor and press P in order to pin.





Create your second seam.





In the object, select the part you want to unwrap again, in the UV Editor remove the pin for this part with AltP.





Unwrap again. In the UV Editor, it won't affect the pinned vertices, it will only affect the non-pinned vertices.





Move and scale the new islands wherever you want in the UV Editor.