using (SPSite oSPsite = new SPSite("http://website_url"))
{
oSPsite.AllowUnsafeUpdates = true;
using (SPWeb oSPWeb = oSPsite.OpenWeb())
{
oSPWeb.AllowUnsafeUpdates = true;
/* Path within the list where the new folder gets created
Leave it empty if it needs to be created under root */
String nodeDepthPath = @"/MyRootFolder";
/* get the list instance by name */
SPList list = oSPWeb.Lists["List_Name"];
/* create a folder under the path specified */
SPListItem folderItem = list.Items.Add(
list.RootFolder.ServerRelativeUrl + nodeDepthPath,
SPFileSystemObjectType.Folder);
/* set the folder name and update */
folderItem["Title"] = "New_Folder_Name";
folderItem.Update();
oSPWeb.AllowUnsafeUpdates = false;
}
oSPsite.AllowUnsafeUpdates = false;
}