This paper investigates the welfare effect of forming a free trade agreement (FTA). To receive tariff‐free treatment, firms must comply with the rules of origin (ROO). Outside firms could undertake either market‐oriented or export‐platform foreign direct investments (FDIs). ROO have the following effects: (i) An infeasible FTA may become feasible by deterring outside firms' FDIs, (ii) an FDI of a less efficient firm could replace that of an efficient firm, or (iii) FDIs made before the FTA is concluded might be eliminated. These potential effects complicate the welfare effect of the FTA and could decrease the consumer surplus.