Due to the scale of nanomaterials, their characterization through experimental measurement cannot be performed by the standard procedure set for macroscopic scale materials. To have a proper guidance on the testing of mechanical properties of nanomaterials, a lot of efforts have been put on their estimation through theoretical and numerical simulation. Most of the simulation methods are based upon molecular dynamics simulation, finite element method, or others. Owing to the vast number of atoms involved in the simulation, it is usually very time consuming. To improve the computational process, recently we proposed a semi-analytical method called molecular-continuum model (MCM). The MCM was developed through the equivalence between potential energy of molecular dynamics and strain energy of continuum mechanics. Potential energy describing the interactions of atoms is not restricted to the harmonic potential function, and hence its deriving stress-strain relation is not restricted to be linear. The estimated properties can therefore be the ones defined based upon the initial linear region such as stiffness, or the ones occur at the later period of the materials such as strength and toughness. Unlike the usual test performed by applying forces, in MCM a uniform strain field is employed in the representative volume element of specimen to estimate the stiffness and strength, and a crack near tip field is employed to estimate the fracture toughness. Through MCM, the Young’s moduli, Poisson’s ratios, and shear modulus of graphene sheets and carbon nanotubes (armchair, zigzag, or chiral) can all be written as a simple rational function in which the dependence of radius, chiral angle and thickness can be observed clearly from the explicit closed-form expression. Although no corresponding explicit expressions have been derived for the ultimate strength and fracture toughness, we still can estimate these values by MCM accurately and efficiently. Furthermore, by extending the concept of MCM, a new approach of multi-scale simulation via coupling of boundary element (for global region) and nonlinear beam element (for near tip region) was proposed to predict fracture toughness and crack propagation of a single layer graphene sheet.