A model of magnetohydrodynamic (MHD) turbulence in accretion disks with mean magnetic fields has been constructed using a second-order closure modeling of turbulence. The transport equations of the Reynolds stress tensor, the Maxwell stress tensor, and the cross-helicity tensor (the correlation of the velocity fluctuation and the magnetic fluctuation) are closed by second-order quantities using a two-scale direct interaction approximation (TSDIA). The quantities appearing in these equations are considered to be those averaged in the vertical direction of the disks; also, stationary turbulence is assumed. We are interested only in the effects of the mean magnetic fields on the turbulence in the disk, i.e., no dynamo processes are considered, and the mean magnetic fields are supposed to be embedded in the disk a priori. The results show that the presence of a radial mean field enhances the turbulent viscosity (and the so-called alpha -parameter value) in the disk. This suggests that the radial mean magnetic field has something to do with advection-dominated disks, in which a rather large viscosity is needed. On the other hand, a toroidal mean field diminishes the strength of the turbulence. Therefore, the strength of the turbulence in the accretion disk is determined by competition between the above-mentioned two opposite effects of the mean fields.