Abstract: A vector hydrophone consists of pressure hydrophone and particle velocity hydrophone, which can co-locately in space and simultaneously in time measures pressure and particle velocity, and obtains acoustic energy flux at a point. In the paper it is shown that in isotropical noise field acoustic energy flux is maximum likelihood ratio detector, its direction is maximum likelihood estimation of source direction. The gain of acoustic energy flux detector and Cramer-Rao Bound (CRB) of Direction of Arrival (DOA) estimation are discussed, and the results of computer simulations and the shallow sea trial are given.