query_posts( array( 'post_type' => array('post', 'portfolio') ) );

query_posts('post_type=portfolio');

<?php
    query_posts(array(
        'post_type' => 'portfolio',
        'showposts' => 10
    ) );
?>
<?php while (have_posts()) : the_post(); ?>
        <h2><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></h2>
        <p><?php echo get_the_excerpt(); ?></p>
<?php endwhile;?>

<?php
defined( 'ABSPATH' ) OR exit;
/* Plugin Name: (#6417) "Portfolio" post type in query */

add_filter( 'pre_get_posts', 'wpse_6417_portfolio_posts' );
function wpse_6417_portfolio_posts( $query )
{
    if (
        ! $query->is_main_query()
        // Here we can check for all Conditional Tags
        OR ! $query->is_archive() // For e.g.: Every archive will feature both post types
    )
        return $query;

    $query->set( 'post_type', array( 'post', 'portfolio' ) );

    return $query;
}

add_action( 'pre_get_posts', 'add_custom_post_types_to_loop' );

function add_custom_post_types_to_loop( $query ) {

if ( is_home() && $query->is_main_query() )

$query->set( 'post_type', array( 'post', 'portfolio' ) );

return $query;

}

add_action( 'pre_get_posts', 'cpt_items' );

function cpt_items( $query ) {

if( $query->is_main_query() && !is_admin() && is_post_type_archive( 'portfolio' ) ) {

$query->set( 'posts_per_page', '8' );

$query->set( 'order', 'ASC' );

    }

}