The Si(100) surface carbonization mechanisms by acetylene are explored using density functional theory calculations combined with microkinetic simulations. The most stable acetylene adsorption geometries and their subsequent decomposition mechanisms to form a carbon dimer on the Si surface are investigated. Microkinetics simulations are further used to examine the optimal reaction conditions for obtaining a single-crystalline silicon carbide (SiC). We find that the carbon dimer (C2*) as an end-bridge structure can be formed at 560 K, and the maximum of C2* can be obtained near 640 K. The acetylene adsorbed via the di-σ configuration starts to dehydrogenate when the heating rate is too fast and will form two possible carbon dimers (di-C2* and C2*), which will lead to a polycrystalline SiC buffer layer. We predict that 750 K and 10-6 bar will be the optimum temperature and pressure for obtaining a single-crystalline SiC buffer layer, respectively.