In the Fast Reactor Cycle Technology Development (FaCT) project, a sodium-cooled fast reactor (SFR) with mixed-oxide (MOX) fuel and an SFR with metal fuel were selected as the primary and the secondary candidates, respectively, for the Japan Sodium-Cooled Fast Reactor (JSFR). The present study focuses on the effects of transuranium (TRU) composition in the design for the JSFR core with MOX fuel. In the transitional stage from light water reactor (LWR) to fast breeder reactor (FBR), there is the possibility for FBR fuel to have high minor actinide (MA) content due to the recycling of LWR spent fuel. High MA content affects core and fuel designs as follows: the neutronic reactivity characteristic changes; the linear power limit is reduced because of decreases of the melting point and thermal conductivity in the fuel; the gas plenum length is extended because of an increase in He gas generation. Thus, to evaluate the effects quantitatively, design studies for cores with two TRU compositions were conducted: an FBR multirecycle composition with [approximately]1 wt% (in heavy metal) of MA content and an LWR recycle composition for which 3 wt% of MA content was assumed as a tentative target. The results show that the change from the FBR multirecycle composition to the LWR recycle composition leads to a sodium void reactivity increase of 10%, a linear power limit decrease of 1 to 2%, and a gas plenum length increase of 5%. As a result, the effects of TRU composition on the core and fuel designs were revealed to be benign.