An analytical formula of the thermal conductivity due to given coexisting electrostatic and magnetic fluctuations is derived in a large aspect straight tokamak with a small gyro-radius. The dependence of the thermal conductivity on particle species and the types of the fluctuations is clearly shown. When the ratio of the electrostatic energy to the magnetic energy is the order of the mass ratio of electrons to ions, χe is governed by the magnetic fluctuations and χi is governed by the electrostatic fluctuations, where χe and χi are thermal conductivities of electrons and ions, respectively.