Applications of ultra short pulses in optical signal processing, communications and high speed electronics have grown rapidly in recent years. However, high stability of the generated pulses is of prime importance for these applications. This paper describes the quantitative stability analysis of optical pulses generated through passively mode locked figure-eight fiber laser operating at fifth harmonic frequency determined by the cavity length. Pulse stability is achieved by introducing an intentional twist in the fiber, thus providing a controllable birefringence-induced phase-bias. The proposed theory has been verified experimentally. The stabilized laser can generate very stable pulses with the wavelengths tunable by simply adjusting the polarization controllers in the two loops of a figure-eight laser. The stability of the laser has been analyzed quantitatively by using RF spectrum analysis of the mode-locked pulse train.