Abstract： Cherenkov radiation (CR) is the electromagnetic wave generated by moving charges passing through a dielectric medium with velocity larger than a certain threshold. To generate CR in natural medium, the electron energy should be as large as hundreds to thousands of keV. We demonstrate in hyperbolic metamaterial that there is no velocity threshold for electron to generate CR. Based on this threshold-less CR, the first on-chip integrated free electron light source was realized. This work opens up the possibility of exploring high performance on-chip integrated free-electron optoelectronic devices.