$b \to s(d) \gamma$ with a Vector-like Quark as Fourth-generation

We study the implications of a vector-like quark, especially a down-type vector-like quark in $b \rightarrow s(d) \gamma$ including the neutral Higgs in the calculation. After analyzing some possible constraints for the mixing, the contribution may be significant for the $b \rightarrow d \gamma$, but not for the $b \rightarrow s \gamma$. We also find that the FCNC among ordinary quarks vanishes for a special form of down-type quark mass matrix.