You should add the following code in functions.php :

add_filter ( 'post_class' , 'my_post_class' );
global $current_class;
$current_class = 'odd';
function my_post_class ( $classes ) { global $current_class; $classes[] = $current_class; $current_class = ($current_class == 'odd') ? 'even' : 'odd'; return $classes;
}

This ensures that all the odd posts on the page will have the class 'odd' and all the even posts will have the class 'even' just by using post_class() in your theme.