The phrase "be tranquil" is often associated with a sense of peace and serenity. There are several words that are synonymous with this phrase, including "calm," "composed," "collected," "cool," "placid," "serene," "soothing," "undeclared," "unruffled," and "peaceable." These words all convey a similar sense of stillness and calmness, and can be used interchangeably with "be tranquil" in a variety of contexts. Whether you are looking to describe a peaceful landscape, a calm temperament, or a tranquil moment in time, these synonyms can help you convey the sense of tranquility you are trying to capture.