Abstract: Vector hydrophone consists of conventional pressure sensor and particle velocity sensor, which co-locately and instantaneously measures pressure and orthogonal components of particle velocity. We use vector hydrophone for measuring noise radiated by the underwater target in low frequency band and give basic principle of the measurement. It is shown that (1) in isotropic noise field, maximum spatial gain of particle velocity is about 4.8 dB, active acoustical intensity is relative to time-bandwidth product; (2) in anisotropic noise field, gain of active acoustical intensity is determined by anisotropy of spatial distribution of noise field.Resultsof computer simulations and the sea trial are also given in the paper.