R natural join S be Relation G(A,B,C)
With FDs A-->B, B--C
'A' will be primary key for G.
For relation to be in BCNF the determinant(left hand side) of all FDs must be super key.
But in this case for FD B-->C , B is not a super key.
Because of this R natural join S is not in BCNF.