In this paper, the author studies the existence of periodic solutions for a second order neutral functional differential equation (phi(p)(x(t) - cx(t - tau))')' = f(x(t))x'(t) + g(t, x(t - mu(t))) + e(t) in the critical case vertical bar c vertical bar = 1. By analyzing some properties of the linear difference operator A : [Ax](t) = x(t) - cx(t - tau) and using Mawhin's continuation theorem, some new results are obtained. (C) 2009 Published by Elsevier Ltd