This is not caused by String.Format()
, more likely it is caused by whatever you use to view the data, or by something that happens before you view the data.
Judging from your format string it looks like you're trying to create a JSON string to return (possibly from a service). There is a big chance something will HTML encode your string on it's way to the client. The problem lays there, not in your string formatting, and thus trying to fix it there will not work.