Abstract: The control systems that use some type of a computer is named computer-based control system. The computer-based system can provide monitoring, intervention control, or direct control of local loop element. It also can provide historical data summaries, data analysis and maintenance scheduling. This thesis introduces one of the control approaches which is ON/OFF control. a closed loop process control system was taken as a case study in this work in which a software program was implemented for monitoring and controlling the hardware suggested design. This application is designed as a prototype implementation for a closed loop process control system. The general idea of the system is to regulate the room temperature at a fixed value to adjust ambient temperature disturbances. The system uses reading values of temperature sensor as the input, then tries to turn on/off the actuator fan/heater using digital output lines of the PC parallel port to get the desired output. This case study discusses and implements the main aspect of the problem, logging temperature measurements, getting desired room temperature and generating the desired control action to the fans and heaters. The whole project is implemented using Windows XP as the operating system. Active Server Pages ASP.Net (C#) is selected as a programming language, using Internet Information Service (IIS), SQL was used as a data server. The system designed and simulated in MATLAB program, different cases were tested to study the heating and cooling, a small and a large difference between the desired and the measured temperature were taken to study the effect of driving different number of heaters/coolers on the response.