Exchange-coupled nanocomposites with hard/soft magnetic phases are promising for the next generation of permanent magnets.Chemical methods have an advantage in controlling the nanoscale size of both phases.The nanocomposites obtained by the chemical method generally consist of a hard phase core and a soft phase shell.However,the soft-phase shell is easily oxidized leading to small enhancement of remanence.Here,a novel microstructure of Fe@FePt nanocomposites with Fe soft phase core and FePt hard phase shell has been synthesized by replacement reaction,in which the size of core and shell can be controlled below 10 nm by adjusting the ratio of Fe nanoparticles to PtCl4.Excellent exchange-coupling(single-phase-like demagnetization curves) between soft-hard phases was observed due to the precise size control of both phases,and substantial enhancements both in remanence (32 ％) and saturation magnetization (81％) were obtained in optimal nanocompistes.This work provides an alternative routine to prepare heterostructure materials with various applications.