ICSHARPziplib library helps you to zip a folder and download it.
go through following link which will help you to zip your folder
ZIP ME[
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and then use following code to download folder
if (File.Exists(szFilePath))
{
HttpContext.Current.Response.AddHeader("Content-Disposition", "attachment; filename=" + fInfo.Name);
HttpContext.Current.Response.AddHeader("Content-Length", fInfo.Length.ToString());
HttpContext.Current.Response.ContentType = "application/zip";
HttpContext.Current.Response.TransmitFile(fInfo.FullName);
HttpContext.Current.Response.End();
return true;
}