After reviewing perturbative QCD, color confinement in QCD is nonperturbatively approached. Gluon propagator in the infrared limit is assumed to be $d(q^2)_{\widetilde{q^2 ~ o}} g^2 \left( \frac{ μ^2}{q^2} \right)^{1- ε}$. Then a differential equation for quark propagator is obtained, which is solved for special case. The bound state problem is well explained.