The self—pulsing behavior of erbium—doped fiber laser has 1seen stuided. Stable-T-periodic and 2T—pe riodic pulse output has been attained and pulse train of random amplitude has been observed in the erbium—doped fiber laser, which was pumped by a semiconductor laser in the CW pumping mode. Considering the change of the photon lifetime of two orthogonal polarization modes, a coupling mode model has been used to analyze the experimental results. The analysis shows that it is important to obtain stable self—pulses by changing the photon lifetime of two orthogonal polarization modes in the erbium —doped fiber laser.