Abstract : A novel triangular resonator with a long evanescent field around the critical angle has been analysed and demonstrated for the first time. The sensitivity of the resonance shift used for changing the refractive index of 1 × 1025 at the incidence angle of 188 is 1.04 × 1027 nm/ RIU for transverse magnetic polarised light. An extinction ratio of 6 dB was observed near 1550 nm, where the incidence angle of the total internal reflection mirror inside the resonator was 188.