//Creating Excel file
private void CreateMailMergeExcelDataFile()
{
try
{
string[] fileds,record1;
 
Object oName = "C:\\TempDoc.xls";
string strHeader = "FirstName, LastName, Address, CityStateZip";
string strRecord1 = "John,Roy,31 New street,320009";
 
MSExcel.Application excelapp;
MSExcel.Workbook excelwrbook;
 
excelapp = new Microsoft.Office.Interop.Excel.Application();
excelapp.Visible = true;
MSExcel.Worksheet ws = new MSExcel.WorksheetClass();
 
excelwrbook = excelapp.Workbooks.Add(objMissing);
 
ws = (MSExcel.Worksheet)excelapp.ActiveWorkbook.ActiveSheet;
 
fileds = strHeader.Split(',');
record1 = strRecord1.Split(',');
//writing in excel you Can use datatable and Get the records and loop.here for
sample i have writing keeping two strings 
 
for (int i = 0; i < j =" 0;" style="color: rgb(0, 102, 0);"  
 //saving the excel workbook
excelwrbook.SaveAs(oName, 
MSExcel.XlFileFormat.xlTemplate, objMissing, 
objMissing, objMissing,objMissing, 
MSExcel.XlSaveAsAccessMode.xlExclusive,
objMissing, objMissing, objMissing, 
objMissing, objMissing);
excelapp.Quit();
 
//opening the excel to act as a datasource for word mail merge
wrdDoc.MailMerge.OpenDataSource("C:\\TempDoc.xls", 
ref objMissing, ref objMissing, ref objMissing, ref objMissing, 
ref objMissing,ref objMissing, ref objMissing, 
ref objMissing,ref objMissing, ref objMissing,
ref objMissing, ref oQuery,ref objMissing, ref objMissing,
ref objMissing);
 
}
catch (Exception ex)
{
MessageBox.Show("Error :" + ex);
}
}
 
private void button1_Click(object sender, System.EventArgs e)
{
try
{
Word.Selection wrdSelection;
Word.MailMerge wrdMailMerge;
Word.MailMergeFields wrdMergeFields;
Word.Table wrdTable;
string StrToAdd;
 
wrdApp = new Word.Application();
wrdApp.Visible = false;
 
// Add a new document.
wrdDoc = wrdApp.Documents.Add(ref objMissing, ref objMissing,
ref objMissing, ref objMissing);
wrdDoc.Select();
 
wrdSelection = wrdApp.Selection;
wrdMailMerge = wrdDoc.MailMerge;
 
// Create a MailMerge Data file using excel 
CreateMailMergeExcelDataFile();
// Create a string and insert it into the document.
StrToAdd = "Mail Merge";
wrdSelection.ParagraphFormat.Alignment =
Word.WdParagraphAlignment.wdAlignParagraphCenter;
wrdSelection.TypeText(StrToAdd);
 
InsertLines(2);
 
// Insert merge data.
wrdSelection.ParagraphFormat.Alignment =
Word.WdParagraphAlignment.wdAlignParagraphLeft;
wrdMergeFields = wrdMailMerge.Fields;
wrdMergeFields.Add(wrdSelection.Range, "FirstName");
wrdSelection.TypeText(" ");
wrdMergeFields.Add(wrdSelection.Range, "LastName");
wrdSelection.TypeParagraph();
 
wrdMergeFields.Add(wrdSelection.Range, "Address");
wrdSelection.TypeParagraph();
wrdMergeFields.Add(wrdSelection.Range, "CityStateZip");
 
InsertLines(2);
 
wrdSelection.ParagraphFormat.Alignment =
Word.WdParagraphAlignment.wdAlignParagraphRight;
 
Object objDate = "dddd, MMMM dd, yyyy";
wrdSelection.InsertDateTime(ref objDate, ref oFalse, ref objMissing,
ref objMissing, ref objMissing);
 
InsertLines(2);
 
wrdSelection.ParagraphFormat.Alignment =
Word.WdParagraphAlignment.wdAlignParagraphJustify;
 
wrdSelection.TypeText("Dear ");
wrdMergeFields.Add(wrdSelection.Range, "FirstName");
wrdSelection.TypeText(",");
InsertLines(1);
 
StrToAdd = "Thank you for using Mail Merge.";
wrdSelection.TypeText(StrToAdd);
// Perform mail merge.
wrdMailMerge.Destination = Word.WdMailMergeDestination.wdSendToNewDocument;
wrdMailMerge.Execute(ref oFalse);
 
// Close the original form document.
wrdDoc.Saved = true;
wrdDoc.Close(ref oFalse, ref objMissing, ref objMissing);
 
 // Makes the merged doc visible
wrdApp.Visible = true;
 
// Release References.
wrdSelection = null;
wrdMailMerge = null;
wrdMergeFields = null;
wrdDoc = null;
wrdApp = null;
 
// Clean up temp file.
System.IO.File.Delete("C:\\TempDoc.xls");
}
catch (Exception ex)
{
MessageBox.Show("Error :" + ex);
}
}