An erbium-doped fiber ring laser with unidirectional operation without optical isolator has been investigated theoretically and experimentally. The fiber ring laser cavity is only composed of two couplers and built in such a way that it offers a higher counterclockwise over clockwise circulating gain ratio, which is strongly dependent on coupling coefficients of the two couplers. The optical fields propagating in the two directions suffer from different losses. As a consequence, the laser oscillation appears in a quasi-unidirectional form without isolators. So, the structure of the fiber ring laser is simplified, eliminating the insertion loss arising from the isolator and decreasing the cost. Two 3 dB couplers are used in the experiment, in which the result shows that on and above the threshold of pump power, there is a difference in the output power of 17 dBm between two contrary directions, basically realizing the unidirectional operation of the fiber laser.