The macroscopic mechanical properties of diamond-like carbon (DLC), which is a promising solid lubricant and protective coating, are directly determined by the atomic-scale structure. Understanding the relation between the atomic-scale structure and the macroscopic properties is necessary and helpful for the improvement of DLC; however, experimental investigations of the atomic-scale structure and macroscopic properties require huge time and economic costs, limiting the progress of elucidating their correlation. In this work, the atomistic simulation approach was used to study the atomic-scale structure of DLC and its effect on the Young's modulus of DLC. Structural analysis of DLC showed that the sp and sp2 hybridizations decrease while the sp3 hybridization increases with increasing density and hydrogen concentration of DLC. The degree of graphitization of DLC was further evaluated and found to be linearly proportional to the sixth power of the sp2 ratio. Then, it was demonstrated that Young's modulus of DLC can be singly predicted by the effective coordination number (CNeff), which is defined as the average coordination number of carbon atoms without the contributions from hydrogen atoms. Furthermore, this work successfully proposed quantitative relations among CNeff, density, and Young's modulus, which were verified by comparison to experimental results.