Nickle ohmic contacts on the Si-face of n-type 4H-SiC are prepared by both rapid thermal annealing (RTA) and laser spark annealing (LSA). The effects of the different annealing procedures on the cathode surface morphology, cathode/substrate cross sectional morphology, element composition, microscopic structure of carbon clusters in the SiC substrate near surface, are characterized by scanning electron microscopy (SEM), atomic force microscope (AFM), transmission electron microscopy (TEM), and Raman spectra, respectively. The tests and analyses show that both thermal treatments can help to form ohmic contacts. The specific contact resistances of RTA sample and LSA sample are measured to be 5.2× 10-4 Ω ·cm2 and 1.8× 10-4Ω·cm2 by transmission line model, respectively. The Ni film of RTA sample shrinks badly thus forms tiny islands on the surface, while the surface of LSA sample remains relatively smooth. The root-mean-square (RMS) values of surface roughness of the Ni films of as-deposited, RTA and LSA samples are 8.65 nm, 91.3 nm and 17.5 nm, respectively. The Ni/SiC interface of RTA sample corrodes badly, and Si can be found in the whole Ni film, indicating an overall consumption of Ni to react with Si forming NiSi compounds; C atoms, which do not react with Ni atoms，cluster to the average size of about 40 Å, and gather approximately as a layer located about 20-30 nm off the Ni/SiC interface. The Ni/SiC interface of LSA sample is relatively smooth, and a small quantity of Ni atoms diffuse into the SiC wafer, forming lots of ternary phase diffusion zones of about tens of nanometers deep into the SiC wafer, in which C, Si, Ni atoms are distributed uniformly; the average size of C clusters is smaller than that in RTA sample and no obvious C enriched zone was found, while neither Si atom nor C atom is found to diffuse into the Ni film.#br#The ohmic contacts prepared by LSA have obvious advantages compared with those by RTA in many aspects such as cathode surface morphology, interface morphology, uniformity of components in cathode films, etc. All the results mentioned above make LSA a promising method of thermal treatment in preparation of ohmic contacts.