This paper presents the dynamic modeling of Mckibben pneumatic artificial muscles. The air flow model of a valve orifice and the air volume model of a pneumatic muscle are incorporated into the proposed dynamic model to estimate precisely the pressure variance of a pneumatic muscle when Mckibben muscles are in inflating and deflating. Coefficient parameters of the proposed model are determined optimally through well-coordinated experiments. Frequency response test is performed on an antagonistic structure consisting of Mckibben muscles manufactured in our laboratory and a pneumatic circuit with fast pneumatic control valves. Comparing with experimental results, simulations revealed that the proposed model gave good performance in estimating motions of the antagonistic actuation as well as the pressure variance of the Mckibben muscles.