Metabolic models have been proven to be useful tools in system biology and have been suc-cessfully applied to various research fields in a wide range of organisms. A relatively complete metabolic network is a prerequisite for deriving reliable metabolic models. The first step in con-structing metabolic network is to harmonize compounds and reactions across different metabolic databases. However, effectively integrating data from various sources still remains a big chal-lenge. Incomplete and inconsistent atomistic details in compound representations across data-bases is a very important limiting factor. Here, we optimized a subgraph isomorphism detection algorithm to validate generic compound pairs. Moreover, we defined a set of harmonization re-lationship types between compounds to deal with inconsistent chemical details while successfully capturing atom-level characteristics, enabling a more complete enabling compound harmoniza-tion across metabolic databases. In total, 15,704 compound pairs across KEGG (Kyoto Encyclo-pedia of Genes and Genomes) and MetaCyc databases were detected. Furthermore, utilizing the classification of compound pairs and EC (Enzyme Commission) numbers of reactions, we estab-lished hierarchical relationships between metabolic reactions, enabling the harmonization of 3,856 reaction pairs. In addition, we created and used atom-specific identifiers to evaluate the con-sistency of atom mappings within and between harmonized reactions, detecting some con-sistency issues between the reaction and compound descriptions in these metabolic databases.