Recent developments in power electronic switching devices and microprocessors have led to power electronic systems with high efficiency and reliability. Today, high performance and low cost are the requirements to be met for most practical systems. This work describes the development of real-time based Uninterruptible Power Supply systems (UPS) which can operate on-line and off-line. Synchronous operation is an inevitable condition in off-line UPS systems. Here, back-synchronisation after a power failure is achieved automatically with the pre-set values. A new modified regular sampled symmetric pulse width modulation method has been developed, it has many advantages especially in low frequency applications. High switching frequency (18 kHz) reduces significantly the filtration and noise problems in UPS systems. Phase delay associated with transformer, filter, load, and control algorithm might cause drastic power failures in by-pass switches, thus to overcome this problem, phase shift control algorithms are developed. PID control algorithm is simplified for real-time on-line operation at high switching frequency. The complete system is based on a single Intel 80C196KC microcontroller chip and the developed software has the flexibility for further development. The developed system has been tested in the laboratory under linear load condition and the results are given. The test results are found to be satisfactory for most load conditions.