function DoCheck(flag)// 全选 flag=1 反选=2 清除=0
    {
        var inputs = document.getElementById("GridView1");
        var chks=inputs.getElementsByTagName("input");
        for (var i=0; i < chks.length; i++)
        if (chks[i].type == 'checkbox')
        {
            if(flag==2)
            {
                if(chks[i].checked)
                {
                chks[i].checked=0;
                }
                else
                {
                chks[i].checked=1;
                }
            }
            else
            {
            chks[i].checked=flag;
            }
        }
   }

 

调用 OnClientClick="DoCheck(1);return false;"