Developing simple and highly efficient technologies for CO2 conversion to chemical fuels is always of great interest. In this work, we report a new and simple method of CO2 conversion into methane in water with an in situ synthesized Ni nanoparticle catalyst, in which the water is used as a hydrogen source and earth-abundant metals (Zn or Fe) are used as re-generable reductants. An excellent 98% yield of methane from either CO2 or HCO3- was obtained at 300 degrees C, and the in situ formed Ni nanoparticle catalyst exhibits not only extraordinary catalytic activity but also stability. Mechanistic studies suggest that the formation of methane from HCO3- or CO2 follows the pathway of HCO3- -> CO2 -> HCOOH -> CH4. This work demonstrates a simple approach for highly efficient conversion of CO2 to methane with earth-abundant materials.