This paper investigates the effects of various R&D investments on economic growth empirically. To this end, the relationships between various R&D investments and economic growth are analysed, and the rates of return of R&D investments are estimated. Furthermore, the effect of government R&D investment on private sector R&D investment, and the effect of social factors, which affect the relationship between the R&D inputs and economic growth, are analysed. Based on the results of this analysis, a simulation model is developed, which shows the relationship between R&D investments and economic growth rate; this model is verified by analysing the correlation between the actual and the estimated economic growth rate, using the data between 1981 and 1995 of eight selected countries. The validation results show that the simulation model has sufficient accuracy to be used for evaluating and proposing R&D policies for the countries for which appropriate data is available. However, the time-lag effect, which is naturally believed to exist between the R&D input and the economic growth, could not be analysed in a mathematical form, because of the lack of the data to establish this relationship. Thus, when estimating the relationship between them, the time-lag effect in this relationship was included implicitly by using the data of fifteen years.