In this picture, I was only masked the area between knees and belt. But when I save the mask as alpha and after call back from the alpha, the mask is coming with some additional parts in shape of lines.
I just want to ask, is this a general problem of zbrush or just my problem?
This does sometimes happen. Especially with a model that has AUVTiles for the mapping. I encountered this with my Select-It plugin, and do not know of a workable solution at this time.
thank you aurick.
Do you know a way to cange the UV map?
Yes. Go to the lowest subdivision level and create a blank texture. Make it 1024x1024, since that is the size that ZBrush will use for the alpha. After that, press Tool>Geometry>GUVTiles.
Your model is now remapped and ready to go.
After you’re done with the sculpting and whatever else you need the stored masks for, you will want to remap your model again before creating texture or displacement maps. Unless, that is, you plan to create the map at 1024x1024 size. The mapping will always be calculated for the size of the currently-selected texture, and won’t work quite as well with other texture sizes.
It worked. Thank you again 
Noo. my mistake. Still there are additional lines but thinner and orderly this time. I’m sure now, it’s exactly an UV map problem.
Can I remade the UV map in max with pelt mapping? I tried to export and import the highpoly mesh but max collapsed. I don’t know what should I do?
There is a video that Cannedmushrooms made that shows some good tips regarding uv’s as well as how to change the uvmapping of your character model in a 3rd party app like you are asking and then apply it to your model. If I remember rightly its in the latter half of the video.
http://www.zbrushcentral.com/zbc/showthread.php?t=37315
Hope this can help you.
Cheers
Mike R
I know this is not a fix to your problem but… you could look on the bright side. You have helped reveal a neat way to make inorganic/cyborg parts. I think I might have some fun with this
Before exporting the mask texture, try expanding the borders a bit. Might help.
Cheers,
R