A novel label-free optical biosensing scheme by using a single mode coupled microcavity laser is experimentally realized. We demonstrate that a slight change of the coupling coefficient due to the existence of biosamples leads to sensitive single mode laser hopping to a new frequency that is several nm away from the original mode wavelength. By monitoring the emergence of the hopped laser mode intensity, the lowest detectable concentration of bovine serum albumin (BSA) of similar to 80 pg/mL was obtained, which is comparable to the detection limit of a passive microcavity sensor with Q > 10(7), but with a much simplified experimental setup. With the mode hopping and mode shift combined, the single frequency coupled cavity laser provides a detection range from pg/mL (Limit of mode hopping) to mu g/mL (Limit of mode shift). Our results show the possibility of using a coupled optical microcavity to achieve ultrasensitive optical sensing.