Thanks to John Magnolia. This is my postForm function that I am using in my Symfony projects and it is fine now to work with CK Editor.
function postForm($form, callback)
{
// Get all form values
var values = {};
var fields = {};
for(var instanceName in CKEDITOR.instances){
CKEDITOR.instances[instanceName].updateElement();
}
$.each($form.serializeArray(), function(i, field) {
values[field.name] = field.value;
});
// Throw the form values to the server!
$.ajax({
type : $form.attr('method'),
url : $form.attr('action'),
data : values,
success : function(data) {
callback( data );
}
});