Abstract : A novel triangular resonator with total internal reflection and attenuated total reflection (ATR) mirrors is proposed. The resonance characteristics are analyzed for the first time. The optimum gold thickness in the ATR mirror is 33 nm and the resonance angle is 17.777. Extinction ratio greater than 66 dB could be achieved with the appropriate choice of resonance angle and coupling ratios. The triangular ring resonator with an incident angle of 18 inside the resonator was also fabricated and measured. The resulting free spectral range of the resonator is approximately 0.85 nm near 1558 nm, and the on-off ratio was about 3.5 dB. These resonance characteristics of the triangular resonator are found to be useful in realizing bio=chemical sensors when the ATR mirror carries a special electromagnetic wave called a surface plasmon wave.