The effects of thickness and temperature on the in-plane thermal conductivities of germanium single-crystal thin films have been investigated by a non-equilibrium molecular dynamics (NEMD) simulation method. The Stillinger-Weber potential model was employed to describe the interaction between atoms in the germanium single-crystal thin films. Taking structural characteristics and heat transfer mechanism of the germanium single-crystal thin films, a steady heat transfer model was framed. The results of calculations demonstrate that the in-plane thermal conductivities of germanium single-crystal thin films show an obvious size effect, which increases with increasing of thin film thickness and decreases with increasing of temperature. Comparing with the cross-plane thermal conductivity, the simulation results prove that the thermal conductivities of germanium single-crystal thin films have anisotropic characteristics.