align an image and some text on the same line without using div width?

Floating will result in wrapping if space is not available.

You can use display:inline and white-space:nowrap to achieve this. Fiddle

<div id="container" style="white-space:nowrap">

    <div id="image" style="display:inline;">
        <img src="https://stackoverflow.com/questions/11713176/tree.png"/>
    </div>

    <div id="texts" style="display:inline; white-space:nowrap;"> 
        A very long text(about 300 words) 
    </div>

</div>​