The phrase "be of no avail" refers to something that is ineffective, useless, or unsuccessful. Some antonyms for this phrase are "be effective," "be useful," "be helpful," "be beneficial," "be fruitful," and "be successful." These terms represent the opposite of being of no avail, as they describe something that has positive results or outcome. For example, when someone puts effort into a task and their hard work pays off, it can be said that their efforts were successful and not of no avail. Antonyms for "be of no avail" can vary depending on the context, but all share the common meaning of being beneficial and effective.