Surface plasmon polaritons (SPPs), the electromagnetic fields propagating along the metal-dielectric interface, are regard as the promising candidate of the next photonic integrated circuits. Several metal slit structures based on the SPPs are summarized. By using the Fabry-Perot resonator effect, Fano resonance, and multimode interference, the SPP propagation could be controlled and modulated by these metal slit structures. As a result, a subwavelength unidirectional SPP generator, a submicron broad unidirectional SPP generator, submicron SPP splitters, nano-focusing devices, and submicron all-optical switches were realized in the metal slit structures. These nano-plasmonic devices have important applications in highly integrated photonics.