Different factors such as nonlinear phenomena and mode instability affect the output of the high-power Fiber lasers and Amplifiers. Since these devices have many applications in the industry, medicine, and military facilities, the study of the various factors’ effects on their output is directly reflected on the design of high-power lasers and Amplifiers. In the present paper, the mode instability which is the major limiting factor on the output of high power lasers and Amplifiers has been studied, simulated, and investigated. In these high-power devices, the temperature increases due to quantum defects, background loss, and light scattering, which change the refractive index of the Fiber material. Variation of the refractive index is the main reason of mode instability in high power Fiber lasers and Amplifiers. Mode instability causes the coupling of the fundamental mode to the upper-mode and thus decreases the fundamental mode’ s energy. In this paper, different factors that affect the threshold of the mode instability and power transfer from the fundamental mode to the upper mode have been investigated.