Changing from the old way of embedding videos to my blog to iframe,
I found that I couldn't autoplay it with just adding autoplay=1, it doesn't work.
I found this new coding here;
https://productforums.google.com/forum/#!topic/youtube/99VJNpUKfIQ
deleting some of the options that I doesn't want, and are as below;
<iframe allowfullscreen="" frameborder="0" height="315" width="420" src="YOUR VIDEO SOURCE?VIDEO_ID&version=4&rel=0&autoplay=1&loop=1&playlist=4v1Li36-0yY" ></iframe>
it work well on pc, but still not autoplaying in smartphones though, but it's good enough for me now. ^^