A life estate cannot be revoked if it is given through a will. A life estate can be revoked if it is given by deed while the grantor was alive.
A life estate is a property right wherein the life estate holder has rights of use and enjoyment of the property but is limited in what she can do with the property. Life estate holders cannot mortgage the property or leave the property to another person in their will. The ownership of the property terminates at the life estate holder's death. Life estates are often used to control who the property goes to after the person holding the life estate dies.