A magnetic nanoparticle applicable in imaging, diagnosis, therapy and biomaterial separation. The magnetic nanoparticle is characterized as comprising at least an inner-transition element, represented as FexMavZy, wherein Ma is an inner-transition element, Z is an element of the group VIa, x is greater or equal to 0, and both v and y are positive numbers. The magnetic nanoparticle may further comprise a shell to form a core-shell structure, wherein the shell is an inner-transition element Mb or the compound thereof.