Finding the best way to show html text in RichTextBlock

Question

I have some string like "Some tmp string with text " or "Some tmp string with text" or with other Html format.

The Requirement is to show this text with the same format in RichTextBlock.

To do this, I tried to make some thing like this:

   <RichTextBlock x:Name="rtb">
            <Paragraph x:Name="par">
                <Run FontStyle="Italic"
                     FontWeight="Bold">
                    Some tmp text
                </Run>
            </Paragraph>            
        </RichTextBlock>

But it formats all the text in Run element, but not the : "Some tmp string with text "

I tried to make Run elements programmatically and add them to paragraph, but have the same problem.

Any suggestions?