in execCommand, How can i know all the available para for the formatblock command?

Question

I'm writting my own rich text editor in javascript

formatBlock requires a tag-name string. I mean all the tag-name strings.

Answer 1

Despite not really knowing what you're after (see my comment), it seems that you want to know more about what is possible with the execCommand method. As usual, the Mozilla docs have a good reference:

Rich-Text Editing in Mozilla

Of course, this only lists the commands that Firefox understands. The list of "command identifiers" that IE supports is given here.