To fill the gap between materials genome and composite structural analysis, a new concept, Structure Genome (SG) is proposed. SG acts as the basic building block connecting materials and structures and contains all the constitutive information needed for a structure the same fashion as the genome contains all the intrinsic information for an organism's growth and development. The Mechanics of Structure Genome (MSG) represents a revolutionary approach to multiscale modeling drastically different from the conventional bottom-up multiscale modeling approaches. The principle of minimum information loss (PMIL) is used to avoid a priori assumptions commonly invoked in other approaches and decouples the original problem into two sets of analyses: a constitutive modeling and a structural analysis. The structural analysis can be routinely performed in commercial FEA software. MSG confines all approximations to the constitutive modeling which can construct constitutive models for all types of structures including 3D solids, 2D plates/shells, and 1D beams, directly linking the structural properties with microstructural details. When specialized to 3D structures, this approach provides a general-purpose micromechanics theory which can provide a better alternative to the RVE analysis and asymptotic homogenization theory in terms of efficiency, accuracy, and versatility. MSG provides a unified treatment for all microstructures (periodic, aperiodic, partially periodic). With MSG, multiscale constitutive modelling is simplified to answer three fundamental questions: 1) what is the original model needed for capturing relevant physics? 2) what is the model wanted for a particular design? 3) what is the SG? With answers of these three questions, MSG allows one to choose the starting scale and ending scale and capture details as needed and affordable.