Path integral molecular dynamics simulation is used to study the rotational motion of a CO molecule doped in a large para-hydrogen (p-H-2) cluster. The quasi-free rotational motion of CO in a p-H-2 cluster with a reduced rotational constant is derived from the imaginary-time orientational correlation functions, and is in good agreement with recent experimental observations. We attribute the reduced rotational constant to the low-viscous fluid-like behavior of the host p-H-2 cluster. (C) 2010 Elsevier B.V. All rights reserved.