Catalytic Chemical Vapor Deposition (Cat-CVD) has been developed to deposit high quality diamond films. However, the reaction mechanism of diamond growth by this technique is still not clear, especially for the diamond (100) surface. Here, semiempirical quantum chemical calculations were used to theoretically study the feasibility of several diamond (100) surface reactions, which have been suggested or are likely to contribute to the growth of diamond. For the different growth radicals, methyl and acetyl, similar surface reactions were studied and the results were compared. The results show that the barriers of abstracting hydrogen are lower than those of adding growth radicals. Also, adding the acetyl barrier is much higher than one for adding methyl.