<p>A theoretical study of the SBS threshold for optical pulses in long optical fibers with and without Modulation Instability (MI) is studied. Using standard equation, the impact of MI on the SBS gain and threshold is examined and to determine the SBS threshold in the presence of MI, an analytical equation is obtained. To study how the repetition rate and Amplified Spontaneous Emission (ASE) affect the SBS threshold for optical pulses, numerical simulation is used. For calculating the threshold power and Brillouin gain with MI and without MI, two equations are derived theoretically. In this study we have calculated transmission power, Brillouin gain and threshold for different fiber length, repetition rate and input power. The results conclusively show how MI impacts optical pulses Brillouin gain and SBS threshold in long optical fibre. The work is beneficial for optical transmission networks and interferometric fibre sensing. The study reports that after around 90&#xa0;km the threshold power saturates though we increase the repetition rate and fiber length keeping the input power same. It is observed that sensing power is activated while the input power is low which in turn lowers the threshold power showing less optical signal loss.</p>