The most commonly used biological treatment of municipal and industrial wastewater is the Activated Sludge Process. The design of activated process is usually done manually; this is often difficult and susceptible to errors due to the large number of equations involved (which call for approximations and round off errors). Also, the determination of the effects of changes in design parameters are very difficult and time-consuming for manual computations. This paper therefore employs the Visual Basic 6.0 programming for design and simulation of completely mixed activated sludge (CMAS) process. Algorithm and flow chart based on kinetic model were used for developing the Visual Basic code, ASPSIM. The code was tested using a design problem from standard text book. Results obtained proves that the Visual BASIC 6.0 program ASP-SIM is a useful tool for predicting the effect of temperature ondesign and operation of completely mixed activated plant design and operation.