Things that you must do is as follow:

SpreadsheetDocument l_package = SpreadsheetDocument.Create(@"file path", SpreadsheetDocumentType.Workbook);

//add a workbook.xml with runtime generated relationship id in /_rels/.rels
var workbookPart = l_package.AddWorkbookPart();
//save the workbook object return by the method to the workbook part

// the return is actually a WorkBook object
GenerateWorkbookPart().Save(workbookPart);

//add a worksheet with relationship id rId1 in /xl/_rels/workbook.xml.rels
//since the part is add to workbookpart1 which is /xl/workbook.xml
var worksheetPart1 = workbookPart1.AddNewPart<WorksheetPart>("rId1");
GenerateWorksheetPart().Save(worksheetPart1);

The GenerateXXXXXXX() can be generate by DocumentReflector tool.

There are two things I want to point out.

1. it seems to me that [Content_Types].xml will be generated when you call SpreadsheetDocument.Dispose().

so, remember to call it or use the 'using' keyword.

2. the example in their manual "How to: Insert a New Worksheet into a Spreadsheet Document by Using the Open XML API " just don't work.

the code as follow has a problem:

uint sheetId = 1;
        if (sheets.Elements<Sheet>().Count() > 0)
        {
            sheetId = sheets.Elements<Sheet>().Select(s => s.SheetId.Value).Max() + 1;
        }
I am using VS2008, the complier will tell you that, no Count() method is being implemented.

and when I want to extend the class by myself using OpenXmlElement.MoveNext() method to count the number of elements.

the MoveNext() method seems to return true no matter what.

I will investigate the MoveNext() method to find out what is going on.

If anyone do know how to count the Sheet elements in Sheets, please tell me.

Thank you.