Related Posts are help your blog to  increase your traffic and Page impressions (views).It is like increase internal link.If people like your related post they will view your post and you will get traffic.Excessive use of plugin may slow your blog, so use good codes to get as option like plugins.To show Related Posts by category without plugin use my super code.I also use it in my blog.

Also Read : Get indexed your blog in Search Engines faster

Related Posts

Also Read : Add Pagination in WordPress without plugin

How to show Related Posts by category without plugin

Paste the code where you want on Single.php .

==========Related Posts by Perfectneed (Start)===================

<?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(); ?>   
<a href="http://www.perfectneed.com/related-posts-by-category-without-plugin" rel="bookmark" title="Show Related Posts by category without plugin in your blog">Related Posts by category</a>
==========Related Posts by Perfectneed (Start)===================

Replace the number how many post you want to show.

'posts_per_page'=> 5,

Before this you need to paste a code in Style.css of your theme .

Also Read : Importance of H1 to H6 tag in posts

Paste this code in style.css file.

===========================================

/*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;
}
===========================================================

After this you can see that Related Posts by category is showing on your blog’s post.