Composite coatings of Al2O3 nanocrystalline/Ni-based powder produced by transverse flow CO2 laser on Ni-based superalloy were introduced. With Olympus Pme-3 type optical microscope, Philip-XL 30 type scanning electronic microscope (SEM) and energy dispersive X-ray spectroscopy (EDS), the microstructure, chemical composition and distribution of nano particles of the cross-sections of the coatings were analyzed. The results showed that columnar dendrite grew perpendicularly to the interface without nano Al2O3 particles. And the growth morphologies of dendrite transformed from long columnar to short form by adding nano additives. Non-directional dendrite growth at the interface and equaxied dendrite across the section of the clad appeared with the additive amount of Al2O3 particles to 1%. The solidified microstructure was greatly refined because the nano Al2O3 accelerated nucleation ahead of the advancing interface of solid/liquid and held back crystal growth by its concentration. Nano particles prevented the formation of cracks in the coatings.