滿神奇的，居然醬子就解決了。

html:

<div class="panel panel-default">
 <div class="panel-heading">
 <h3 class="panel-title pull-left">
With short text it works great!
 </h3>
 <button class="btn btn-default pull-right">New</button>
 <div class="clearfix"></div>
 </div>
 <div class="panel-body">Panel content</div>
</div>

css:

.panel-heading h3 {
 font-weight: 400;
 width: 75%;
 padding-top: 8px;
 font-size: 14px;
}