We numerically study surface-plasmon (SP) mediated semiconductor light-emitting diodes (LEDs) and show that mediation of SPs can be useful for high power LEDs in their modulation speeds and directionalities. It has been reported that SPs can drastically enhance internal quantum efficiencies and speeds of InGaN quantum-well (QW) LEDs by letting them dominate spontaneous emission (SE) processes. Many experimental and theoretical studies have been conducted in this context but most of the works have dealt SE into SPs and light extraction from excited SPs separately. In particular, there is no theoretical analysis, to our knowledge, which simultaneously considers SE into SPs on a textured metal surface along with its extraction to outside radiation. In this presentation, we numerically study InGaN QW LEDs which consist of 1-D metallic gratings on a p-contact electrode and an adjacent single QW emitting green light by using finite-difference time-domain (FDTD) method. We focus on the case where the first order diffraction of SPs produces lightwaves propagating along the surface-normal direction. SP band-edge effect on SE rate, extraction of SPs into internal-radiation, and angular directionality of final outside-radiation are analyzed. Practical enhancement of LED performances are discussed on the basis of the simulation results.