Try this solution (From MSDN)
ExportOptions exportOpts = new ExportOptions();
ExcelFormatOptions excelFormatOpts = new ExcelFormatOptions ();
DiskFileDestinationOptions diskOpts = new DiskFileDestinationOptions();
exportOpts = Report.ExportOptions;
excelFormatOpts.ExcelUseConstantColumnWidth = true;
excelFormatOpts.ExcelTabHasColumnHeadings = true;
exportOpts.ExportFormatType = ExportFormatType.Excel;
exportOpts.FormatOptions = excelFormatOpts;
exportOpts.ExportDestinationType = ExportDestinationType.DiskFile;
diskOpts.DiskFileName = fileName;
exportOpts.DestinationOptions = diskOpts;
Report.Export ();
Pay special attention to the line:
excelFormatOpts.ExcelTabHasColumnHeadings = true;