I want to make some words in epub on ipad stay as fixed font size.
No matter I use ipad to change the font size to be bigger or smaller, I want them stay as fixed font size.
How to do it? Or this is impossible?

You would have to embed it as an image I suppose otherwise it will change when the reader changes font size.

In theory, for a well-behaved reader, you could just set a fixed font size:

<p>In a normal paragraph, <span class="fixed">these words have fixed font size</span>, and the rest is variable.</p>

span.fixed { font-size: 12pt; }