A novel oscillating field biosensor was proposed utilizing ultrahigh-order modes excitated in a symmetrical metal cladding waveguide (SMCW). Different from the conventional biosensors, such as surface plasmon resonance sensor (SPRS), leaky mode waveguide sensor (LWS) and reverse symmetry waveguide sensor (RSWS), the sample is placed inside the guiding layer. Since the guiding layer supports a high intensity oscillating field, the intensity of output light is rather sensitive to the tiny variation of refractive index, which leads to very high sensitivity and resolution. Based on the noise level, 1ppm variation of the glucose solution was detected in the experiment. The proposed scheme is expected to have a wide application and promising future in various biomedical detections.