I'm attempting to upload video to Wordpress.using following method

string expression = videoData.VideoTitle;
string str7 = videoData.VideoTitle;    
WebClient client = new WebClient();
client.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
string s = "security_code=JGZpbGVu&post_content=<iframe width='480' height='390' src='http://www.youtube.com/embed/XkIdmC8MlkI' frameborder='0' allowfullscreen></iframe>"
                    + "&post_title=" + expression + "&post_category="
                     + "&post_category=" + category + "&category=" + category + "&post_name=" + str7 + "&post_date=" + DATE_STRING;
byte[] bytes = Encoding.ASCII.GetBytes(s);
byte[] buffer2 = client.UploadData(uploadPath, "POST", bytes);
string str8 = Encoding.ASCII.GetString(buffer2).ToString();

it really works and things are published but the one thing in tags doesn't get published i attempted some few things what switched out is it had issues with tag. after i remove < of iframetag it's displayed but till url and then things are gone. here's the one thing which doesn't works well

"security_code=JGZpbGVu&post_content=test iframe width='480' height='390' src='http://www.youtube.com/embed/XkIdmC8MlkI' frameborder='0' allowfullscreen></iframe>&post_title=How to build, upgrade, or repair, your own PC Computer&post_category=&post_category=asd&category=asd&post_name=How to build, upgrade, or repair, your own PC Computer&post_date=2011-05-11 00:00:00"

any idea what's wrong?

You'll most likely have to URL-encode the HTML.

Make certain you reference System.Web, then:

string html = @"<iframe width='480' height='390' src='http://www.youtube.com/embed/XkIdmC8MlkI' frameborder='0' allowfullscreen></iframe>";
string s = string.Format("security_code=JGZpbGVu&post_content={0}&post_title={1}&post_category={2}&category={3}&post_name={4}&post_date={5}", System.Web.HttpUtility.UrlEncode(html), expression, category, category, str7, DATE_STRING);