var jsonstr='{"value":"abcd<efg"}';
$.post(
url,
{ "jsonstr": jsonstr },
function (data, status) {
});

transferredChars=function (htmlChars) {
var tcs = htmlChars.replace(/</g, "<");
tcs = tcs.replace(/>/g, ">");
return tcs;
}

var jsonstr='{"value":"abcd<efg"}';
jsonstr=transferredChars(jsonstr);
$.post(
url,
{ "jsonstr": jsonstr },
function (data, status) {
});