A wavelength-tunable CO2 laser (spectrum range from 9.2 to 10.9 μm) was applied in a multi-energy process for growing diamond crystals in open air. A pre-mixed C2H4/C2H 2/O2 gas was used as precursors for the diamond growth. Laser energy was coupled into the reactions through resonantly exciting the CH2-wagging mode of ethylene (C2H4) molecules by tuning the laser wavelength to 10.532 μm. Diamond growth rate and diamond quality were both promoted by the laser-induced resonant excitations. High-quality diamond crystals were grown on silicon substrates with a high growth rate of ∼139 μm/hr. Diamond crystals up to 5 mm in height and 1 mm in diameter were grown in open air in 36 hours. Sharp Raman peak at 1332 cm-1 with a full width at half maximum value around 4.5 cm -1 and distinct X-ray diffraction spectra indicate the high quality of the diamond crystals.