function ck_click(obj) {    var ck_20 = document.getElementById("ck_20");    var ck_25 = document.getElementById("ck_25");    var ck_30 = document.getElementById("ck_30");    ck_20.checked = false;    ck_25.checked = false;    ck_30.checked = false;    obj.checked = true;} <input type="checkbox" id="ck_20" value=http://www.mamicode.com/"20" name="ck" onclick="ck_click(this)">                    <input type="checkbox" id="ck_25" value=http://www.mamicode.com/"25" name="ck" onclick="ck_click(this)">                    <input type="checkbox" id="ck_30" value=http://www.mamicode.com/"30" name="ck" onclick="ck_click(this)">