problem to convert special character in export to csv

I am using FIleStream object to write .csv file from dataTable/Dataset.
It is working fine. But when i got special character in my DataTable like $, @, # ect. it will convert these convert in encoded form( which can't be readable i.e.  â,¬ etc).
 

FileStream flStream;
astrFileName = HttpContext.Current.Server.MapPath("~/files/FtpFiles/Download/") + astrFileName + ".csv";
if (File.Exists(astrFileName))
{
flStream = new FileStream(astrFileName, FileMode.Truncate, FileAccess.Write);
}
else
{
flStream = new FileStream(astrFileName, FileMode.Create, FileAccess.Write);
}
StreamWriter objSw = new StreamWriter(flStream);
DataTable objDt = idtInstance;
//Get No Of Column in GridView
int NoOfColumn = objDt.Columns.Count;
//Create Header
for (int i = 0; i < NoOfColumn; i++)
{
objSw.Write(objDt.Columns[i]);
//check not last column < ...