Google released +1 buttons to the whole web. As a result, you might start seeing +1 appear on sites large and small across the Internet. Google partnered with a few sites where you’ll see +1 buttons in the near future.

With a single click you can recommend that news article or favorite sci-fi movie to friends, contacts and the rest of the world. The next time your connections search, they could see your +1’s directly in their search results, helping them find your recommendations when they’re most useful.

So if you want to add google +1 button for each post on your wordpress blog just add following lines of code inside your themes functions.php file

Add Google Plus one (+1) button without plugin

<?php
/* Add plusone.js to <head> section of your theme  */
add_action ('wp_enqueue_scripts','google_plusone_script');
function google_plusone_script() {
wp_enqueue_script('google_plus_one', 'https://apis.google.com/js/plusone.js', array(), null);
}
/* Add +one button after your post content by using filter */
add_filter('the_content', 'google_plus_one');
function google_plus_one($content) {
$content = $content.'<div class="google_plus_one">
<g:plusone size="tall" href="'.get_permalink().'"></g:plusone></div>';
return $content;
}
?>

You can style and align button in post text by applying following class

Add this code in your theme’s style.css if you want to align it to left

.google_plus_one{
display: inline;
float: left;
margin-right: 20px;
margin-top: 4px;
}

And add this code in your theme’s style.css if you want to align it to right side

.google_plus_one{
display: inline;
float: right;
margin-left: 20px;
margin-top: 4px;
}

For more details about the options and Google Plus One Button Visit
Google Plus One Button