Experimental method and multiscale modelling based on molecular dynamics simulation have been used to determine mechanical properties of carbon nanotube and graphene nanoplatelet hybrid nanocomposites. Samples containing two different mix ratios of hybrid nanofillers were subjected to tensile loading. Also, micrographs were obtained from samples fracture surfaces using field emission scanning electron microscope and SEM. These images showed that nanofillers were well dispersed in the matrix. In multiscale modelling, a new method has been developed to estimate the Young's moduli of hybrid nanocomposite samples. To this end, atomistic modelling has been employed to investigate the mechanical properties of molecular samples at nanoscale level. In this step, epoxy cross-linked atomic models have been constructed utilizing molecular dynamics. Next, micromechanical modelling was used to bridge elastic constants of molecular samples from nanoscale to microscale. Also, the calculated Young's moduli of the hybrid nanocomposite samples obtained from multiscale modelling were compared with experimental measurements and differences less than 7% were observed. Finally, the effects of effective fibre aspect ratio on elastic modulus of hybrid nanocomposite were investigated using the multiscale method. The results indicated an increase in hybrid nanocomposite Young's modulus with effective fibre aspect ratio. In summary, this paper presents a new method for calculating the hybrid polymer-based nanocomposite properties using a multiscale method.