Abstract : In this letter, a triangular resonator-based surface plasmon resonance (SPR) sensor that uses total internal reflection (TIR) mirrors to provide SPR excitation for the detection of refractive index (RI) change is proposed and first demonstrated using SoI substrate without any gain medium. The fabricated triangular resonator sensor with a 35-nm-thick Au film on a TIR mirror shows an output power increment of 1.2 dB and a sensitivity of 102 nm/RIU, while that nonmetallic layer on a TIR mirror shows 30 nm/RIU. The sensitivity enhancement is attributed to a phase change in the SPR resulting in a large shift of the resonance peak. This configuration provides significant advantages by increasing the sensitivity of the RI sensor even with the extremely small sensing area of the SPR mirror.