Abstract : This thesis deals with the analyses of UPS system and how the output voltage of the UPS can be stabilized at a certain Level, also it studies the performance parameters of the converters included in the UPS system (rectifier and inverter). For the three-phase bridge rectifier, the harmonic components present in the output voltage waveform of the three-pulse, six-pulse and twelve-pulse are investigated. It is found that the orders of the generated harmonics are the same, but the amplitudes of these harmonics decrease with increase of number of pulses. It is also found that the best performance (low harmonics) can be obtained when the firing angle is set to zero. By using a lowpass filter with specific design, the generated harmonics can be eliminated or reduced. The amplitudes of the harmonic components and the output voltage waveform of the rectifier are drawn using MATLAB program. For single-phase inverter controlled by PWM technique, the optimum PWM strategy is used and investigated to control the output voltage of the inverter. From the mathematical analysis and computer simulation, a technique for stabilizing the output voltage of UPS system is proposed in the present work. Analysis of this technique reveals some equations which need to be solved to get best performance. Two methods are used to solve these equations, first by using Newton_s method, and second by using system identification. The second method gives better solution than Newton_s method. However, both methods yield approximate solutions. But, by using a curve-fitting technique a linear relationship between the fundamental voltage (output voltage) and triggering angles (a) can be obtained and it gives an exact solution. A Flowchart is presented to describe the operation of DSP processor which can be used to control the output voltage of the inverter. A MATLAB program is used to draw the amplitudes of the harmonic components and the output voltage waveform of the inverter