  The relative Grothendieck group $K_0(\m V/X)$ is the free abelian group
generated by the isomorphism classes of complex algebraic varieties over $X$
modulo the "scissor relation". The motivic Hirzebruch class ${T_y}_*: K_0(\m V
/X) \to H_*^{BM}(X) \otimes \bQ[y]$ is a unique natural transformation
satisfying that for a nonsingular variety $X$ the value ${T_y}_*([X
\xrightarrow {\op {id}_X} X])$ of the isomorphism class of the identity $X
\xrightarrow {id_X} X$ is the Poincar\'e dual of the Hirzebruch cohomology
class of the tangent bundle $TX$. It "unifies" the well-known three
characteristic classes of singular varieties: MacPherson's Chern class,
Baum-Fulton-MacPherson's Todd class (or Riemann-Roch) and Goresky-MacPherson's
L-class or Cappell-Shaneson's L-class. In this paper we construct a bivariant
relative Grothendieck group $\bK_0(\m V/X \to Y)$ so that it equals the
original relative Grothendieck group $K_0(\m V/X)$ when $Y$ is a point. We also
construct a unique Grothendieck transformation $T_y: \bK_0(\m V/X \to Y) \to
\bH(X \to Y) \otimes \bQ[y]$ satisfying a certain normalization condition for a
smooth morphism so that it equals the motivic Hirzebruch class ${T_y}_*: K_0(\m
V /X) \to H_*^{BM}(X) \otimes \bQ[y]$ when $Y$ is a point. When $y =0$, $T_0:
\bK_0(\m V/X \to Y) \to \bH(X \to Y) \otimes \bQ$ is a "motivic" lift of
Fulton-MacPherson's bivariant Riemann-Roch $\ga_{td}^{\op {FM}}:\bK_{alg}(X \to
Y) \to \bH(X \to Y) \otimes \bQ$.
