The transient current dynamics of multilevel nanoscale systems weakly coupled with two electrodes has been theoretically investigated by the nonequilibrium Green’s function method. We have proposed a technique for decomposing the total time-dependent current into almost independent current components. This is a powerful technique for gaining insight into transient current behavior because the decomposed currents exhibit simple behaviors similar to those of single-level systems. We have clarified the transient current behaviors of a hydrogen molecule (two-level system) and an octatetraene molecule (eight-level system) connected to two electrodes as typical examples of multilevel systems. The transient current of the molecular hydrogen system is characterized by two current components with the same relaxation time and different oscillation periods, whereas the current of the octatetraene molecular system is decomposed into eight components, a few of which dominate the total current behavior during the initial stage.