export to excel, number problem

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C#

MS-Excel

Gridview

StringWriter strWriter = new StringWriter();
HtmlTextWriter htmlWriter = new HtmlTextWriter(strWriter);

Response.Clear();
Response.Buffer = true;
Response.AddHeader("content-disposition", "attachment;filename=excel.xls");
Response.Charset = "";
Response.ContentType = "application/vnd.ms-excel";

i used above code to convert my gridview to excel, everything is good except one field which contains up to 12 numbers.
they look good in gridview, after convert to excel, it became:

   Gridview        Excel
860201231356 to 8.60201E+11
123456121115 to 1.23456E+11
123456120010 to 1.23456E+11

i need to manually
1. Change the format from text to number
2. Press "Decrease decimal" to remove the ".00"

if there anyway to solve this?