This study aims to clarify the influence of cobalt (Co) diffusion on the chemical vapor deposition (CVD) process of hydrogen-terminated diamond (100) surfaces, with particular attention to its effects on dehydrogenation reactions and the adsorption behaviors of critical carbon-hydrogen (C-H) groups. Currently, pretreatment methods are commonly employed to remove cobalt from the substrate in order to mitigate its effects during diamond deposition. However, these methods tend to reduce the substrate’s toughness and increase preparation costs. Moreover, even if cobalt is only partially removed, residual cobalt within the substrate can still diffuse into the film-substrate interface and the diamond film during the deposition, thereby degrading the quality of the diamond film. The main goal of this study is to examine, on an atomic scale, how cobalt atoms diffusing into the diamond substrate affect the key reactions during diamond growth, specifically, dehydrogenation and C-H group adsorption. Understanding these effects is essential for developing strategies to mitigate cobalt’s adverse influence on diamond deposition.Using first-principles calculations based on density functional theory (DFT), we construct geometric models of single-crystal diamond and its (100) surface. Co atoms are introduced at various diffusion depths (ranging from the 2nd to the 5th layer beneath the diamond surface), and the surfaces are hydrogen-terminated to simulate experimental conditions.The Dmol3 module in Materials Studio is used to simulate and analyze the energy barriers for dehydrogenation reactions and the adsorption energies of key C-H groups, which include CH, CH_2, CH_3. Transition state searches are performed to identify reaction pathways and energy profiles, and adsorption energies are calculated to assess the stability of C-H group binding at active sites. The presence of Co significantly increases the energy barriers for dehydrogenation reactions.The extent of this increase is positively correlated with the projected distance ( D_\textCo\hbox---\textH ) between surface H atoms and Co atoms. Additionally, although the number of layers separating Co from the surface also affects the energy barrier, this influence is less significant than that observed in the case of D_\textCo\hbox---\textH . Co diffusion changes the adsorption energies of C-H groups, particularly increasing the adsorption energy of CH₃, a pivotal group in diamond growth. This results in reduced adsorption efficiency of CH₃, thereby degrading the quality of diamond deposition. The influence varies with Co’s diffusion depth: at the 2nd layer, all C-H groups exhibit increased adsorption energies, indicating thermodynamic instability; at deeper layers (3rd to 5th), CH₃ consistently shows higher adsorption energies compared with Co-free conditions, while CH and CH₂ display more complex behaviors, with some layers showing reduced adsorption energies. Our findings provide crucial insights into the atomic-scale mechanisms by which cobalt affects diamond CVD. The significant increase in dehydrogenation energy barrier and the changed adsorption behaviors of C-H groups, especially CH₃, underscore the challenges in depositing high-quality diamond films on WC-Co substrates. These results guide the development of strategies to mitigate cobalt’s adverse effects, such as optimizing substrate pretreatment or inserting barrier layers, ultimately improving the quality of diamond films on cobalt-containing substrates.