Perfect Related post by Category without plugin

<?php $orig_post = $post;
global $post;
$categories = get_the_category($post->ID);
if ($categories) {
$category_ids = array();
foreach($categories as $individual_category) $category_ids[] = $individual_category->term_id;
$args=array(
'category__in' => $category_ids,
'post__not_in' => array($post->ID),
'posts_per_page'=> 5, // Number of related posts that will be displayed.
'caller_get_posts'=>1,
'orderby'=>'rand' // Randomize the posts
);
$my_query = new wp_query( $args );
if( $my_query->have_posts() ) {
echo '<div id="perfect-related_by-category" class="clear"><h3>Related Posts</h3><ul>';
while( $my_query->have_posts() ) {
$my_query->the_post(); ?>
<li>
 <a href="<?php the_permalink()?>" rel="bookmark" title="<?php the_title(); ?>">
 </a>
 <div class="perfect-related_by-category">
 <a href="<? the_permalink()?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a>
 </div>
</li>
<? }
echo '</ul></div>';
} }
$post = $orig_post;
wp_reset_query(); ?>


Also put this code in style.css

/*Perfect Related post by Category without plugin*/

.custom #perfect-related_by-category ul {
 margin:0;
 overflow:hidden;
}
.custom #perfect-related_by-category li {
 float: left;
 list-style: none;
 margin: 0 0 0 20px;
}
.custom #perfect-related_by-category li:first-child {
 margin-left: 0;
}
.custom #perfect-related_by-category li a {
 display: block;
 font-size: 12px;
 line-height: 16px;
 text-align: left;
 text-decoration:none;
 width: 110px;
}
.custom #perfect-related_by-category img {
 border: 2px solid #DDDDDD;
 height: 110px;
 width: 110px;
}
.custom #perfect-related_by-category li a:hover {
 text-decoration: underline;
}