  It is challenging to quantify chaos of QCD, because non-perturbative QCD
accompanies non-local observables. By using holography, we find that QCD
strings at large $N_c$ and strong coupling limit exhibit chaos, and measure
their Lyapunov exponent at zero temperature. A pair of a quark and an antiquark
separated by $L_q$ in the large $N_c$ QCD is dual to a Nambu-Goto string
hanging from the spatial boundary of the D4-soliton geometry. We numerically
solve the motion of the string after putting a pulse force on its boundaries.
The chaos is observed for the amplitude of the force larger than a certain
lower bound. The bound increases as $L_q$ grows, and its dependence is well
approximated by a hypothesis that the chaos originates in the endpoints of the
QCD string.
