I've followed several related solution. But I'm still facing the problem.
I've applied the following code for my custom post query by taxonomy (category). But it's not working. I think my problem with 'terms'.

                $terms = get_terms('service_cat');
                $services_post = new WP_Query(array(
                    'post_type'         => 'service',
                    'posts_per_page'    => -1,
                    'tax_query' => array(
                        array(
                            'taxonomy' => 'service_cat',
                            'field' => 'cat1',
                            'terms' => array($terms)
                        )
                    ),
                ));
I also tried by the following code

        $services_post = new WP_Query(array(
                    'post_type'         => 'service',
                    'posts_per_page'    => -1,
                    'category_name'     => 'cat1' 
        ))