The aim of this study is to estimate the function and volume of FDI in Iran. Since data related to FDI are often inaccurately recorded or reported less than the actual amount, the fuzzy systems method estimates this variable. If the answers obtained from fuzzy estimation are similar to the existing results, this method can be used to obtain data on the FDI variable and predict it in future years. For this purpose, two variables, gross domestic product and real exchange rate, were used in the period 1986-2022, and then the FDI variable was. The results showed that the trend of this variable is the same as the reported trend, so that the fuzzy method can be a suitable method for estimating and predicting the size of the FDI variable accurately and realized. Also, to estimate the Iranian FDI model, an autoregressive model with distributional lags was used. The results showed that the variables GDP, real effective exchange rate, import tax, as well as lags of inflation rate and cost of capital employed are significant in the short run. Also, in the long run, all the variables in the model are statistically significant.