Bongvireak

The HTML

We need two divs, one of them is the container and the other wraps the text:
<div id="container">
    <div class="content">
        Your text goes here
    </div>
</div>

Replace the dummy text with your own.

The CSS

The CSS will point to the two elements from the previous step:
#container {
    display: table;
    height: 400px;
    margin: 0 auto;
    width: 400px;
}
.content {
    background: #f2f2f2;
    padding: 40px;
    text-align: center;
    display: table-cell;
    vertical-align: middle;
}

As you can see, we use properties to "emulate" a table. We use display: table for the container, and display: table-cell, plus also vertical-align: middle for the text wrapper.

Optional: you can customize the values for these properties:

background
padding
text-align


The end result

Preview the result to see the text is perfectly centered vertically: