  Since the CP violation in the $B$ system has been investigated up to now only
through processes related to the $B$--$\bar{B}$ mixing, urgently required is
new way of study for the CP violation and establishing its origin in the $B$
system independent of the mixing process. In this work, we explore the
exclusive $ B^0 \to \rho^0 \gamma$ decay to obtain the time-dependent CP
asymmetry in $b \to d$ decay process in the standard model and the
supersymmetric model. We find that the complex RL and RR mass insertion to the
squark sector in the MSSM can lead to a large CP asymmetry in $b \to d \gamma$
decay through the gluino-squark diagrams, which is not predicted in the
Standard Model induced by the $B$--$\bar{B}$ mixing.
