The phase properties of the field in the Jaynes-Cummings model with an additional Kerr medium are discussed from the point of view of the new phase formalism of Pegg and Barnett. The phase probability distribution, the expectation values and the fluctuations of the Hermitian phase operator and the phase fluctuations are obtained. The effect of nonlinearity interaction of Kerr medium on the phase properties are analysed in comparison with the standard Jaynes-Cummings.