A novel multimode interference-based refractive index sensor in a higher-order-mode-no-core-higherorder- mode fiber structure achieved by shining a zero-order Bessel-Gauss beam is reported here. The effect of higher-order mode coupling on the performance of the proposed sensor is investigated and verified numerically. Based on the simulation results, the proposed sensing scheme offers a very high sensing resolution of 5.54 × 10-6 RIU over the refractive index range of 1.33-1.42 for a no-core fiber length of 15 mm. So, the sensor we proposed has significant advantages in the field of any physical, biological, and chemical sensing purposes as it provides measurement with very high sensitivity.