string obj = "<b><i>The is test string</i></b>";
Regex regexObj = new Regex("\\<[^\\>]*\\>");
Response.Write(String.Format("<b>was: </b>{0}", obj));
// the HTML Text
obj = regexObj.Replace(obj, String.Empty);
Response.Write(String.Format("<b>now: </b>{0}", obj));
// Plain text as output

this program finds the Regex string and is replaced
with empty string.to make it more clear in
understanding,suppose a string is - "<li>test</li>"
then after the program the output will be simple
"test" as plaintext.