In this work, we design a new biosensor concept that uses a microcavity in 2D photonic crystal (PhC) to sense small refractive index (RI) changes. The RI sensor is formed by a point-defect resonant cavity in the sandwiched waveguide with triangular lattice of air holes. The properties of the sensor are simulated using the finite-difference time-domain method. The calculation results show that a change in ambient RI is apparent; the sensitivity of the sensor is achieved. We succeeded to obtain a new sensitivity value of 425 nm/RIU with a detection limit of 0.001 RIU, which proves the ability of the structure to produce biosensor PhC.