Reliability and function of the technology are two important aspects of the computer controlled systems. But in the concrete design

people often concentrate more on the function of the technology than the reliability

especially the software reliability. This ignorance leads to the system out of control and results in the economic loss

even endanger people`s life. This article introduces the computer controlled system reliability in quantity. The effects of software reliability and hardware reliability on the systems reliability are analyzed. The methods of solving the problems are discussed.