Is it OK for the supervisor to expect half of grant money which student got through his help? My supervisor has told all his PhD students (more than 20) that from now on if a student gets grant then half of it must be given to group so that it could be used for the expenses for other students like conference travel. I have not applied for any grant and at the moment I am not planning to. I just want to know if this is ethical?

What I feel is that supervisors are there to supervise and it is their job. They are paid for it. If they want money, apply for grant by themselves, why expect money from a poor student? 

Please not that the grant which the student gets is specifically for students. And supposed to be applied only by students themselves.
No, this is not o.k. Moreover, depending on the grant we are talking about, this instructor's request may even be illegal, but it most certainly is unethical. From what you write, it appears that in fact the request is illegal. I realize that this may be difficult, but you may want to discuss this issue with the department chair.