many people say it is a opensource soft,
but why can’t i find it source in the network?
Kindly use the Search functionality next time 
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It’s not open-source.
OpenGL defines an API. So, an interface. Not an implementation.
There are multiple implementations of this API. Most of them, provided by the graphics cards manufacturer are closed source.
However, Mesa has an MIT license, so it is open source: http://www.mesa3d.org/
More details for the different licenses for the different components of Mesa here:
http://www.mesa3d.org/license.html
Get the code of the releases here:
http://www.mesa3d.org/download.html
Get access to the GIT repository here:
http://www.mesa3d.org/repository.html
Heh, the OpenGL being an API is pretty open (not a source/soft though).
Check out the specification: http://www.opengl.org/documentation/specs/