  In this paper we study the dynamical instability of Sakai-Sugimoto's
holographic QCD model at finite baryon density. In this model, the baryon
density, represented by the smeared instanton on the worldvolume of the probe
D8-\overline{D8} mesonic brane, sources the worldvolume electric field, and
through the Chern-Simons term it will induces the instability to form a chiral
helical wave. This is similar to Deryagin-Grigoriev-Rubakov instability to form
the chiral density wave for large N_c QCD at finite density. Our results show
that this kind of instability occurs for sufficiently high baryon number
densities. The phase diagram of holographic QCD will thus be changed from the
one which is based only on thermodynamics. This holographic approach provides
an effective way to study the phases of QCD at finite density, where the
conventional perturbative QCD and lattice simulation fail.
