//Checked 상태를 리턴하는 함수
Function TForm1.GetCheckState(hItem:HTREEITEM):Boolean;
const
  TVM_GETITEMSTATE=TV_FIRST+39;
begin
  Result:=Boolean(SendMessage(TreeView1.Handle,TVM_GETITEMSTATE,
                  LongInt(hItem),TVIS_STATEIMAGEMASK) Shr 13);
end;

//Checked 상태를 설정하는 프로시져
procedure TForm1.SetCheckState(hItem:HTREEITEM;State:Boolean);
var
  TVi:TTVITEMEX;
begin
  TVi.mask:=TVIF_STATE;
  TVi.hItem:=hItem;
  TVi.stateMask:=TVIS_STATEIMAGEMASK;
  TVi.state:=ord(GetCheckState(hItem)) or ((ord(State)+1) Shl 12);
  SendMessage(TreeView1.Handle,TVM_SETITEM,0,longInt(@TVi));
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  SetCheckState(TreeView1.Items.Item[0].ItemId,True);
end;