Effects of blanket composition, including materials and their thicknesses, on the tritium breeding ratio in tokamak fusion reactors are investigated for the Li20 blanket having a separable first wall. The sensitivities of the breeding ratio to the thicknesses of the materials for the first wall are estimated as follows (unit: TBR/cm): Ssic= −.05, Scu= −.13, SAl= −.04, Sss= −.03, SHe= 0.0, SD2o= −.02, SH2o= −.09. From these results, aluminum and stainless steel are seen as suitable for such first-wall structural materials as cooling tubes, and heavy water is appropriate for the coolant of the first wall. The lead multiplier of 5-cm thickness is used along with Li20, without 6Li enrichment, as the tritium breeding material. The tritium breeding ratio of the blanket is estimated as 1.08.