Abstract: Using unidirectional ring resonator is the most promising way to get high power single-longitudinal-mode (SLM) laser output. To enforce the singlelongitudinal-mode ring cavity to operate unidirectionally, a component which can introduce loss difference between the counter-propagating beams in the cavity is necessary. A diode-pumped Nd∶YAG SLM four-mirror rectangle ring laser was experimentally demonstrated. The stability of the cavity was ensured by the thermal of the gain medium, and an intra-cavity acousto-optic modulator was used as the optical diode, which provided different losses on the counterpropagating beams. The beam with larger loss died out in their competition for the gain and hence the unidirectional operation was demonstrated. An SLM output power of 1 W at 1.06 m with a beam quality factor M2 of 1.21 was obtained. The considerable increased output power and compact architecture are especially important for future practical application.