Electrically pumped lasing action has been realized in ZnO from an n-MgZnO/i-ZnO/SiO2/p-Si asymmetric double heterostructure, an ultralow threshold of 3.9 mA was obtained. The mechanism of the laser is associated with the in-plane random resonator cavities formed in the ZnO films and the elaborate hollow-shaped SiO2 cladding pattern, which prevent the lateral diffusion of injection current and ultimately lower the threshold current of the laser diode. In addition, a waveguide mechanism due to different refractive indices of three epilayers enhances the guided optical field on the ZnO side, resulting in an improved light extraction efficiency.