[html]
<object width="425" height="344">
<param name="movie" value="http://www.youtube.com/v/XXXX&hl=ja&autoplay=1"></param>
<embed src="http://www.youtube.com/v/XXXX&hl=ja&autoplay=1" type="application/x-shockwave-flash" width="425" height="344"></embed>
</object>
[/html]
URL に “&autoplay=1” をつければ OK