//Create workbook object Workbook wb = new Workbook("source.xlsx"); //This is how to save Workbook to XLS format MemoryStream ms1 = new MemoryStream(); wb.Save(ms1, SaveFormat.Excel97To2003); //Just to check if memory stream is good and contains XLS bytes byte[] b1 = ms1.ToArray(); File.WriteAllBytes("output.xls", b1); //This is how to save Workbook to XLSX format MemoryStream ms2 = new MemoryStream(); wb.Save(ms2, SaveFormat.Xlsx); //Just to check if memory stream is good and contains XLSX bytes byte[] b2 = ms2.ToArray(); File.WriteAllBytes("output.xlsx", b2);