Computer Systems which operate in space environment are Subject to different radiation phenomena that lead to soft errors and can cause unpredictable behaviours of computer-based systems. Commercial Off-The Shelf (COTS) equipment which is commonly used in space missions cannot tolerate some threats such as Single Event Upsets (SEU). Therefore, there are some considerations in resisting this equipment against possible threats. In this paper, a software instruction level method that is called Soft Error Detection using Redundant Instructions (SEDRI) is provided to detect soft errors which influence control flow and program data. This method is evaluated by fault injection on several C benchmark programs. The experimental results show that without protecting a program against control flow and data errors 34% of them affect the program and damage it; but, by using our method, this rate is decreased to about 11%. Comparing to previous presented techniques, SEDRI method has a considerable improvement in performance and memory overhead, ie, 46% and 55% respectively, and its fault coverage decrease about 9%.