One loop corrections to the quark propagator are calculated in massless QCD with a domain-wall fermion. We adopt the Shamir type domain-wall fermion with an infinitely large extra dimension to describe the massless fermion. It is shown that no additative counterterm to the current quark mass is generated in this theory with infinitely many flavors, and the wave function renormalization factor of the massless quark is explicitly evaluated. We also show that an analysis with a simple mean-field approximation can explain the properties of the massless quark in numerical simulations of QCD with domain-wall quarks. © 1999 The American Physical Society.