HEAs have unveiled and unique mechanical responses and need constitutive equations based on deformation mechanisms and microstructural features, instead of simple stress-strain numeric data. In this presentation, even if many factors governing properties of HEAs are still unclear, we propose constitutive models based on microstructural features, such as grain size, dislocation density, twin probability, friction stress, and texture, and deformation mechanisms, such as dislocation glide, twinning, and diffusion. Experimental works for mechanical behaviors, e.g. indentation, static tensile testing, and dynamic shock testing, and for microstructural characterizations using synchrotron XRD, SEM-EBSD, TEM, and 3D atom probe tomography, are performed in order to correlate microstructural features with mechanical responses, and finally to develop the mesoscopic mechanism-based constitutive model. The proposed model is verified by investigating mechanical responses and by quantitative comparisons of predicted and experimental features of microstructure.