As a novel strategy of designing alloys, the concept of multiple-principal elements alloys has promoted an emerging development of alloys in undiscovered compositional areas. As a result, multiple-principal elements alloys have been reported to show various beneficial properties such as high fracture toughness, high-temperature strength, excellent strength-ductility balance as outstanding structural materials. However, until now most of the research has focused on understanding and optimizing their mechanical properties for structural applications. In this dissertation, we used this alloy design approach to soft magnetic and thermoelectric materials. Especially, we used advanced characterization such as atom probe tomography and transmission electron microscopy to understand the structure-property relationship. As a result, we revealed the effect of nanostructures and their compositions on their functional properties. We expect that the correlative characterization approach proposed in this dissertation could be competently used for revealing the structure-property relationships in wider field of materials science.