This　paper　constructs　a　three-dimensional　cellular　automaton　for　simulating　GBM　tumor　growth　(CASTG).　With　extended　Moore　neighborhood,　the　CASTG　unifies　the　rules　for　evolution　of　the　proliferative　cells,　the　non-proliferative　cells　and　the　necrotic　cells.　The　cells　and　neighborhoods　and　rules　of　the　CASTG　completely　abide　the　designed　principle　of　standard　cellular　automata　so　that　virtual　experiment　and　analysis　on　the　CASTG　can　be　carried　out　under　the　unified　theoretical　frame　of　cellular　automata.　The　results　of　simulating　GBM　tumor　with　the　CASTG　were　consistent　with　the　clinical　data　taken　from　the　medical　literature.