Whispering-Gallery-Mode (WGM) microresonators have shown great promise for ultra-sensitive and label-free chemical and biological sensing. The linewidth of a resonant mode determines the smallest resolvable changes in the WGM spectrum, which, in turn, affects the detection limit. The fundamental limit is set by the linewidth of the resonant mode due to material absorption induced photon loss. We report a real-time detection method with single nanoparticle resolution that surpasses the detection limit of most passive micro/nano photonic resonant devices. This is achieved by using an on-chip WGM microcavity laser as the sensing element, whose linewidth is much narrower than its passive counterpart due to optical gain in the resonant lasing mode. In this microlaser based sensing platform, the first binding nanoparticle induces splitting of the lasing line, and the subsequent particles alter the amount of splitting, which can be monitored by measuring the beat frequency of the split modes. We demonstrate detection of polystyrene and gold nanoparticles as small as 15 nm and 10 nm in radius, respectively, and Influenza A virions. The built-in self-heterodyne interferometric method achieved in the monolithic microlaser provides a self-referencing scheme with extraordinary sensitivity, and paves the way for detection and spectroscopy of nano-scale objects using micro/nano lasers.