Metastable high entropy alloys (HEAs) have attracted extensive attention due to their excellent combination of high strength and great plasticity. In this work, we utilize constraining effects to tailor phase transformation strengthening and plasticity of nanostructured HEA ([double bond, length as m-dash]FeCoCrNi) thin films prepared by the magnetron sputtering technique via HEA/Cu and HEA/Ni nanotwinned nanolaminates (NTNLs). It is uncovered that the HEA/Cu NTNLs without phase transformation exhibit the fashion of "smaller is stronger" and HEA layers become more favorable to detwin at a smaller layer thickness (h). By contrast, the HEA/Ni NTNLs manifest an ultra-high peak hardness plateau accompanied by the FCC-to-HCP phase transformation in HEA layers at large h≥ 25 nm, whereas they manifest size-dependent hardness when detwinning occurs at smaller h. This unusual plastic deformation behavior of HEA/X (X = Cu, Ni) NTNLs was rationalized by partial-based mechanisms. These findings open a new avenue to achieve superior mechanical properties of HEAs particularly at the nanoscale.