This paper focuses on the stress-strength reliabilities of basic multi-component system designs which are frequently studied in the literature such as series, parallel, consecutive G. Suppose that the distribution of the strength variable of a component is unknown and approximated by RSM by considering specific factors that affect the strength variable. Firstly, the stress-strength reliability of a single component is obtained under a certain assumption, and then the stress-strength reliability of multi-component system designs are investigated individually under two assumptions on stress variable. These assumptions can be stated as A.1 (e.g., there is a common random stress effect upon each component in the system) and A.2 (e.g., there are different stress effects upon each component in the system). The concerned system reliabilities for two assumptions are expressed in general terms. Besides, system reliabilities are obtained for the second assumption in case the stress variable conforms to normal distribution. Two examples about inventory systems are presented in order to use the proposed method in obtaining the distribution of the strength variable and to determine design points that give the optimum system reliability. A simulation study has been carried out for the required data set to be used in these examples.