Thanks joe. It works.
I found another way for this. It applies new easyui tooltip on every cell of specified datagrid.
This is the code:
var cells = $('#mygrid').datagrid('getPanel').find('div.datagrid-body td[field] div.datagrid-cell:not(:empty)');
cells.tooltip({
position:'top',
content: function() {
var tp = $(this).html();
return tp;
}
});